For starters if you really want to find out what HCG does then dont rely on answers you get here, spend some time googling it and reading various medical studies as to what it does.
HCG mimics LH. LH signals the testicals to release testosterone. When you take AAS your body reduces LH output from pituitary and thus your testis dont make any test and usually shrink because they arent doing anything. I personally would use HCG all through a cycle to keep testis producing test then stop its use as the AAS is leaving the body. That way your not battling pituitary AND testicular shutdown, just pituitary shutdown.
If the above doesnt make sense to you then your not ready for AAS and need to spend more time learning how things work.