HCG is Human Chorionic Gonadotropin which in males, it acts like LH. Now im not going to get indepth for spermatogensis but its a key part of sperm formation. When you get down to it, if you can create functional sperm then you have proper hormone levels, plain and simple. This is a balance between Testosterone , estrogen, progesterone, and cortisol. This is the steroid hormone balance, there is also some thyroid hormones that play a part in this also, but since those arent affected by steroids very much (except for Tren , which a noob shouldnt take and by the time they are ready for it they should have a functional knowledge of how this all works) Im not gonna go into that.
When you put exogenous testosterone in the body, it stimulates the inhibition of production of GnRH (LH + FSH) which travel down to the testes to produce sperm. This process is inhibited when estrogen values get to high in the blood then the Pituitary sees this and stops secretion. This estrogen is derived from higher estrogen conversion because you have more test in your body. This inhibits the secretion of LH and FSH which then doesnt travel to your testes and they dont produce testosterone. That is the basic reason why your balls shrink on cycle because they arent producing test. They lose the Intratesticular testosterone volume in the testes which with out this they shrink down. They arent producing test, so they basically go back to prepubertal size before they produce testosterone and sperm in the quantity post pubertal. So essentially what you are doing while on cycle is 'reverting' back to prepubertal state of hormone production but still benefiting the gains of testosterone that you supply exogenously. Then PCT is essentially a real quick start up of puberty again and try to get producing your own test again while your body is in a state of hormone deprivation. Thats why PCT is so important and there is a whole sub-forum on it with several stickies.

HCG acts like LH when you have exogenous test in your system so that keeps your testes going. Although there isnt any FSH, the HCG is sufficient to keep spermatogensis going while you are on cycle (1). Remember you inhibit your own production of sperm from both levels of stimulation (pituitary and testes) (which Hypogonadotropic (no LH FSH) and hypogonadal (shriveled testes)(HGHG) This shows that HCG is capable of stimulating spermatogensis even without the presence of FSH. Now that in the study it was natural HGHG not induced by a steroid cycle, so we can assume that there is a natural functioning spermatogensis before a cycle and with HCG usage on cycle you can maintain spermatogensis which helps recovery ALOT! if you keep rolling instead of stopping its much easier to get back rolling if you dont stop.

So in a real quick explanation, ( its out of my free time, dont have incentive to go gung-ho with many sources and thorough explanation) HCG is best for ON CYCLE.

Another thing, it should NOT be used daily. The testosterone effect resulting from HCG is about 72 hours. (2) So by using this daily, its going to be a waste and since it is a hormone that works in a diurnal fashion (meaning its like a wave, goes up and down; this is natural in the body) If you take this ED you dont have it used by the body correctly which can lead to LH recpetor insenstivity, which is very bad for recovery because since natural LH is suppressed following a cycle you want the receptor to be as sensitive as possible.

One issue with HCG is it does increase aromatization seperately so you will have a greater conversion to estrogen. However, IGF-1 (potent growth factor) is dependent on estrogen you should only take precautions if you start to have issues. If you are gyno prone, you should be on Nolva on cycle and if have other issues should think of starting a AI along with the nolva. (Nolva is best for gyno only, using a AI wont always take care of the problem, but thats a different issue)

So for a quick dosage:
250iu - 500iu 2x a week ON CYCLE, up untill your first day of PCT. So if using test e, you still take it in the 2 weeks between last shot of test e and start of PCT.
Now you definately want to run this at the end of your cycle at bare minimum of 5 weeks, but since HCG is cheaper compared to the other stuff you gotta get on cycle it shouldnt be a problem.
I would begin HCG on the 2nd week of any cycle and run it up untill first day of PCT, starting at 250iu 2x a week and then you have to pay attention to your body. If you are still having some small testes then up it to 500iu but you have to give it some time to fully work (so 1-2 weeks)


http://www.eje-online.org/content/147/5/617.short (1)
Maintenance of spermatogenesis in hypogonadotropic hypogonadal men with human chorionic gonadotropin alone
http://www.ncbi.nlm.nih.gov/pubmed/16285473 (2)
Serum estradiol after single dose hCG administration correlates with Leydig cell reserve in hypogonadal men: reassessment of the hCG stimulation test

^2 important sources, but if you question the validity of something else i said just lemme know and ill try to get the article back