How does HCG (human chorionic gonadotropin or human chorionic gonadotrophin) raise testosterone? Structurally, HCG is very similar to LH (Leutinizing Hormone). And, in fact, HCG's testosterone superpowers come from the fact that the body thinks it is LH. If you'll recall, LH is sent from the pituitary and signals the testes to make testosterone. Therefore, HCG also acts as a signal to your testicles to pump out more T.
Is HCG "natural"? To answer that question, you have to remember that the primary pulses for LH, come during the early morning hours during sleep. HCG, on the other hand, is given by injection and lasts in the system only for a few days, gradually diminishing based on its half life. So, although HCG is a natural molecule produced by the placenta and utiltized during birth, it really is not administered in the same way that your body is used to receiving LH. But then neither is HRT and it has done very well in the studies to date in my opinion.
Of course, HCG has been widely used in weight loss treatments (of debatable value) as I outline here in my link on HCG and Weight Loss. And, in males, it is used medically for pubertal issues, such as undescended testes, and fertility. However, there has even been an "underground" use of HCG for years to help steroid users recover their testosterone production.
Many, if not most, steroid users push their testosterone levels significantly beyond normal physiological ranges and end up shutting off virtually all natural production. The excess estrogen and DHT can cause a host of other issues as well, including gynocomastia ("bitch tits"), leaky/irritated nipples, water retention and acne! They have learned all kinds of tricks to help them get around these issues, but, in the case of shrunken testicles and low T production, they often have to resort to treatment with HCG..
One good thing that came out of all these steroid recovery treatments was a greater understanding of other ways that HCG might be used. And, from what I can tell, more and more physicians are actually using it for treatment. Consider what these forum posters wrote recently:
--"If the Clomid doesn't do it for me, he said we'd try HCG." 
--"He put me on a quick protocol of HCG stimulation which would provide a higher LH."
And now HCG is being used for a wide variety of off label applications. But it is interesting how many ways one molecule can be utilitzed. Here are four of the more interesting ones:
1) Testicular Atrophy. HCG is sometimes given in order to induce short term stimulation of the testes and testosterone, generally in order to combat testicular atrophy that comes from steroids or overly aggressive testosterone therapy.
Generally, testosterone therapy (HRT) does not result in significant testicular shrinkage, especially the topicals (Androgel, Testim, compounding). However, all testosterone therapy will tend to shut off the feedback cycle that occurs from the hypothalamus/pituitary to the testes to one degree or another. Essentially, the body says, "Oh! I've got plenty of plasma testosterone" and, therefore, does not pump out as much leutinizing hormone. (The feedback is actually based on estradiol interesting enough, but estradiol rises with increasing testosterone due to aromatization.) Of course, this is why hormone replacement therapy will almost always negatively affect male fertility. Again, though, testicular shrinkage should be minimal if dosed correctly.
However, I do occasionally receive complaints from men on injections, because injections start a man off at very high testosterone levels which then taper off to very low levels at the end of the cycle. HCG may be a possible solution for a man that has experienced a significant loss in testicular size from this phenomenon. Discuss with your physician.
2) Testing for Secondary Hypogonadism. Some knowledgeable doctors will actually test for secondary hypogonadism (low testosterone resulting from pituitary or hypothalamus dysfunction) using a short regimen of HCG. The idea is that, if HCG can stimulate the testes to produce testosterone, then more than likely the gonads are still functional and okay. Finding a doctor that will even care or bother to look for secondary hypogonadism is difficult at best, however.
Does HCG raise testosterone in all men? The answer is no and goes back to the mechanism by which it works. Again, HCG mimics LH (Leutinizing Hormone) and thus "tricks" the testes into producing more testosterone. Unfortunately, this does not work well with men who have primary hypogonadism, i.e. dysfunction of the Leydig cells in the testes. You can't "squeeze blood out of a turnip", eh?
HCG works best in this fashion with men who have some kind of issue with the pituitary or hypothalamus, rather than with the testicular cells that actually produce testosterone. This is why it is so popular with steroid users, who are most mostly younger males who have not yet developed traditional andropause and loss of testicular function. Almost all of the men on the Peak Testosterone Forum are secondary hypogonadal.
Did you know you can inexpensively do your own testing for many hormones? The industry leaders is Discounted Labs..
NOTE: It is important to find a good doctor when going on HCG. Too much HCG can lead to over-aromatization, i.e. too much testosterone converting into estrogen. Thus, solid experience and monitoring are in order. Extreme cases can eve result in a kind of burn out of the testicular production of testosterone.
3. Fertility. HCG is used by many fertility doctors to help with fertility. For example, look what this forum poster wrote: "Definitely look into clomid and/or HCG to recharge/super charge your fertility again... Clomid took my sperm count numbers from very low into the way high end, and it improved the quality. My urologist said HCG (which i'm on now) is even more powerful/good for sperm.."  Again, traditional HRT will almost always lower fertility, whereas HCG can boost testosterone and maintain or even improve fertility a little. (Check with your physician of course on something as important as fertility!) The reason for this is that HCG's LH-mimicking abilities kickstart the testes into action and sperm is generally increased according to some fertility docs and urologists specializing in the subject.
4. Correction (Someday) of Secondary Hypogonadism? Apparently, there are a few doctors that believe they can actually solve low testosterone in some cases using HCG as the initial protocol. Of course, this cannot work on a man with primary hypogonadism where the testes themselves are simply not functioning properly. However, secondary hypogonadism can in some cases be caused by extreme stressors, such as trauma and (some experts believe very strong) emotional events. Could the "switch" somehow be switched back on using an HCG protocol?
One forum poster outlined how this worked for him: 
"The first reading I got from him in the morning was 498. Keep in mind during this time I had been getting readings from other doctors and all of them were around 305-363 ng/dl and also in the morning as well. I actually got checked into the hospital for a severe depressed episode and had a very sympathetic psychiatrist test my testosterone at around 6pm and it came to 199 ng/dl ( hypogonadal for almost any lab and yes I felt every bit the part of those low levels that many of us on this forum have come to unfortunately know). So the doctor I'm seeing prescribed me a quick regimen of HCG for four weeks. My levels per his lab went from 498 to 699 and this reading was taken in the afternoon because my new job currently requires my early mornings and cannot schedule a morning draw."
My leutinizing hormone was rock bottom because the hcg supressed its secretion. Remember HCG mimicks LH in the body and will definately have an effect on the HPTA. After the HCG the doctor prescribed me nolvadex or tamoxiphene citrate (selective estrogen receptor modulator) at 20 mg per day for 4 weeks. Nolvadex is used to fight breast cancer in women but it also has an effect on blocking the pituitary from sensing any estrogen in the body and this in turn stimulates the hypothalamus to send messages to the pituitary to secrete LH. The labs taken after the nolvadex showed 701 ng/ml and again this was taken in the afternoon ( 4:30pm) where a man's levels are definately not peaked. The LH reading was slightly over the normal range and this was due to the nolvadex really getting the pituitary to work."
"Finally now after a month the doctor had another blood draw in the afternoon from me to see where the levsl were at after the nolvadex had left my system and the lab came to 625 ng/ml. This is very promising as the doctor feels it may be staying there and he thinks I might have a level around the high 700's or low 800's perhaps in the morning. I am definately not near as depressed and the libido has improved."
So, according to this poster, a month afterward, the treatment had "stuck" and appeared to have been successful. Does this mean that secondary hypogonadism can really be cured in some cases using these kind of "post-steroid recovery strategies"? Well, this user later lost his gains and ended back where he started from. But some steroid users have been able to heal post-cycle, so perhaps this will be possible in the future. Hopefully, some of the anti-aging and sports medicine physicians will test and develop strategies that can help some men with their secondary hypogonadism. Right now, though, it is much more "art than science."
5. HCG Monotherapy. Some men are just leaving out the testosterone altogether and sticking to just straight HCG (and Arimidex almost always). I discuss this option in my link on HCG Monotherapy.
Remember: almost all of these usages are off label and have little study work behind them. With HCG you are almost always relying on the expertise and judgement of your physician, so get a good doc with good experience. Remember that more is NOT better with HCG and the knowledgeable docs advocate low dosage HCG. If you go high enough with HCG, it is common knowledge that it can desensitize receptors.
NOTE: One interesting potential benefit of HCG is the adrenal connection. Leutinizing Hormone actually stimulates, at least in men with low adrenal function, the cholesterol to pregnenalone pathway and thus may help some men in later stages of adrenal fatigue. 
This is the protocol that my current HRT clinic uses and the pattern seems fairly common:
About 85% of the men at my clinic are on HCG and around half say that they feel some kind of significant positive effect. How much will HCG increase your testosterone? This is highly variable and varies between about 50-300 ng/dl according to the information that I was given. Of course, this does depend on the dosage you are given.
NOTE: HCG has a 24-hour half life and so this protocol only has significant HCG in your system for maybe 72 hours. However, this is enough to keep the testes alive and most men with shrinkage will see an increase in testicular volume from this.
NEWS FLASH: Dr. Lipshultz has several studies under his belt showing that most men on HRT can maintain their fertility if they add in low dose HCG. One study put men either on injections or daily transdermal (topical) testosterone gels and then added in 500 IU every other day of HCG. The abstract states the remarkable results that "no impact on semen parameters was observed as a function of testosterone formulation. No patient became azoospermic during concomitant testosterone replacement and human chorionic gonadotropin therapy. Nine of 26 men contributed to pregnancy with the partner during followup."  Fertility is so important that I encourage you to talk to fertility specialist and come up with a treatment plan that will work for your situation rather than just go by this one study. But, yes, it looks like you can have your cake and eat it too!
4) "AN UPDATE TO THE CRISLER HCG PROTOCOL", By John Crisler, DO
5) 6) The Journal of Urology, February 2013, 189(2):647 650, "Concomitant Intramuscular Human Chorionic Gonadotropin Preserves Spermatogenesis in Men Undergoing Testosterone Replacement Therapy"