There's no need to clarify how TH procs progress. That is well known despite the confusion of some people in this thread (who the hell really cares if TH2 replaces TH1 or adds to it to make TH=2). What is not known clearly is the effect that the various levels of TH have on drop rate. I don't think that SE will ever tell us how this actually works because if they do, everyone would realize that beyond TH3 or so, it makes such a small difference as to be completely insignificant and be very pissed.