Answer
Your nuts shrink basicly b/c while you are on cycle as a safety mechanism, the body stops producing test. It pretty says your already have too much in the system and to compensate for this the body stops producing it.
A month or so after your cycle depending on the severity of your cycle, they will return to normal and you will start producing test agian. Most bodybuilders use PCT to help their body gradually start producing test. This helps you retain more of your gains and have a shorter down time(body is w/out test). This is a simplified explanation, but it is many theories and much more complicated. It is some who even do PCT(w/HCG) thoughout their whole cycle to have the natural test high and synthetic test at the same time theorizing that with both their gains will be double. This argumentative to a degree b/c logically your body will have a greater shutdown when you come off and A/R receptors will dull b/c of the presence of so much test. The simple fact is that your body will only use but so much test at one time before it becomes detrmental and causes aromatization and androgenic side effects.
Well hope this answered most of your questions.
Walk the Path of Iron and you will be fine, Iron Brother!!
