Oh someones gonna come on and say Im wrong or not exactly right but its just because Im rusty and dont feel like googling it again , but HCG is a fertility drug derived from the urine of pregnant women. It affects the body in a similar way that clomid does by stimulating the bodys natural feedback loop to produce testosterone and sperm. That part is not in question, its the exact mechanism I forget. I believe it stimulates the pituitary by mimicing leutenizing hormone which in turn triggers the body to activate the testes and produce more sperm. Bottom line is it does do that, but like I said Im rusty and may not be spot on in the way it does it.
Uses for HCG while on cycle or TRT are to attempt to keep your balls from completely shutting down and thats arguable as to whether or not that works. I believe it does and have gotten my wife preggers twice while fully on blast so something is still functioning down there. Plus the majority of TRT clinics recommend the use of HCG The other use is as a transitional drug before you enter pct to give your self a jump start. But do not use during pct.
Some argue that HCG use causes excess estro conversion and I have not had that to be the case unless extreme doses are used. I find it a very useful drug and in fact if you crash your estro an extreme HCG blast could help you bounce back better, as crashed estro sucks.
You dont want to run HCG forever as you will become desensitized to it, 3-5 week runs is it with at least that in between. You can however alternate between HCG and clomid since the do the same thing but act on different mechanisms in the loop.