Testosterone production is a chain reaction starting with the hypothalamus, which signals the pituitary, which signals the testes (the HPTA).
HCG mimics lueteinizing hormone which is the chemical signal from the pituitary to the testes. Thus it causes the testes to produce testosterone but it suppresses the hypothalamus and pituitary in the same way that AAS does.
So, HCG is used during a cycle to keep the testes at full size and function. Or, alternately, just after a cycle to restore the testes to full size and function. Because HCG is syppressive to the hypothalamus and pituitary it's use in either of the methods above should be followed by 3 weeks of S.E.R.M. (clomid or nolvadex) therapy.