Endogenous testosterone production is a chain reaction of the HPTA (hypothalamic pituitary testicular axis):
1.) The hypothalamus produces GnRH (gonadotropin releasing hormone)
2.) This stimulates the pituitary to produce LH (luteinizing hormone)
3.) Which stimulates the testes to produce testosterone.
High levels of sex hormones (testosterone, estrogen, progesterone, prolactin) inhibits activity at the hypothalamus which reduces GnRH production which ultimately leads to lower endogenous testosterone production. This is known as the negative feedback loop.
After about 3 weeks on 250mg/week or more of AAS, the entire system is shut down completely. The purpose of PCT is to get the HPTA back to full function as quickly as possible so you don't lose your gains due to low serum testosterone levels.
A SERM (Selective Estrogen Receptor Modulator) like Clomid or Nolvadex can block estrogen receptors at the hypothalamus and interupt the negative feedback loop. This hastens and increases the production of GnRH which is the first step in the chain reaction of testosterone production.
Studies show that the hypothalamus and pituitary can recover fairly quickly even after years of testosterone replacement therapy. However, the testes can be the bottleneck in the recovery process and the longer they are dormant, the longer they will take to recover.
This is where HCG comes in; it can act just like LH and stimulate the testes to produce testosterone. However, this increased testosterone inhibits the hypothalamus through the negative feedback loop so timing of HCG use is critical.
As a rule of thumb, after any cycle of 10 weeks or longer, HCG use is warranted to speed up recovery.
One option is to use it weekly throughout your cycle so that your testes are never dormant. However, care must be taken not to use it at too high a dosage or for too long as it can permanently reduce the testes sensitivity to LH and ultimately impair your ability to produce endogenous testosterone.
The othe (more common) option is to use HCG the last 3 weeks of a cycle before PCT begins. For instance, if your cycle included EQ or deca, you'd use 1,000IU HCG EOD for 3 weeks immediately following your last injection.
In another example, if enanthate was the longest ester in use during your cycle, you'd use the same 3-week HCG protocol but begin it one week before your last injection.
Using HCG in this way will acomplish 2 things:
1.) It will restore the testes to full function in preparation for PCT so your overall recovery will be quicker.
2.) It will keep your serum testosterone levels elevated while the AAS esters are working their way out of your system so you spend less time overall with lower serum test levels.
But, as I stated before, HCG is just as suppressive to the hypothalamus and pituitary as AAS are. So after the 3 week protocol of HCG, you still need to go through the standard 3 week PCT with a SERM like clomid or nolvadex.