And don't go automatically thinking that when your on trest your infertile. It takes 3-4 months for sperm to fully develop. So any sperm your body starts making the day you start taking trest will continue to develop to full maturity. That's why you hear stories of guys knocking up their gals in the middle of a cycle. In fact, my now 6 month old daughter was conceived during week 6 of an SD cycle (we were trying and it was going to be my last cycle for a while).
Anyways, one of the other reasons trest was being researched as a fertility drug for men was that bounce back from trest was very quick. So, although it will shut down your nuts hard and fast, for some reason you bounce back quickly as well. And the reason it is recommended as a test base on a cycle (and another reason it was being looked at as a fertility drug) is because it is very biologically similar to test and your body functions pretty normally on it.