Because HCG is an LH analogue, the body thinks it's procuding too much and suppresses its release from the Pituitary. Since this is already suppressed from the exogenous test, it's a moot point. The fact is that once you stop taking the HCG your test levels will drop off again until the hypothalamus and pituitary begin to produce sufficient GnRH and LH/FSH levels. The recovery for H and P are much faster than the testes so HCG can improve recovery but while you are on it, it causes suppression so you can not fully recover.