Pnova has a CHANCE of resetting the counter, if you look at poison nova's damage it is variable between a range. If your second cast is lower damaging/per sec than your previous cast, it will do nothing, however, if your second cast is higher damaging/per second, it will reset the timer with a more damaging pnova. The damage variable is minimal however, so more than three novas is a waste IMO. (I generally only cast one!)
Spamming nova is good for one reason though, it makes everyone in a baal run quite angry with lag, that's useful if they have angered you first
He may want to switch to amp for skellies or CE MM
