AIs are used on a cycle to avoid steroids aromatization wich would lead to gyno, bloaty look, higher suppression, ect... HCG is to mantain leydig cells function (if you use it on cycle you avoid harsh suppression, because it sends signs to the testicles to keep working)... if you use at the last weeks of your cycle or on PCT, would be in order to make your testicles recover theyre function (since they would be atrophied).... so in plain englisgh HCH on cycle keeps your testicles going and avid strong suppression.... HCG on pct is to recover... if you ask me i prefer the first use (on cycle)