Testosterone production is a chain reaction where the hypothalamus produces gonadotropin-releasing hormone (GRH) which signals the pituitary to produce luteinizing hormone (LH) which signals the testes to produce testosterone.
After about 3 weeks of AAS use (at body building dosages) this entire system is shut down. During recovery, the hypothalamus recovers first, then the pituitary, and finally the testes. Basically it takes about 3 weeks for the pituitary to recover and produce the normal amount of LH. If the testes have atrophied, they can take another 3-4 weeks to recover after LH levels have returned.
HCG can mimic LH and stimulate the testes to produce testosterone, but it will suppress the hypothalamus and pituitary just the same as AAS. So there are two proper uses of HCG:
During cycle to keep the testes from atrophying in the first place
or
The last 2 or three weeks that AAS are in your system, to recover testicular function in preparation for clomid/nolvadex to recover the hypothalamus/pituitary.