Quoted from Djshakes:Seems the upper flipper engages on a full button push, not a half push. Lower engages on half. Again, I'm probably being too picky.
Has to do with the opto interrupters. There are two different lengths on the interrupter for the two different opto circuits of the two different flippers. The one for the bottom flipper "uninterrupts" the beam first, then the one for the upper.
Same operating principle as the more mechanical version of the double flipper switch found in a TAF.
The pros practice finding that spot(where the upper flipper engages) on each machine they play in a tourney to be able to trap a ball below, but still use the upper for the jackpot shots. It's quite the thing to watch!