Quoted from wisefwumyogwave:

The ball release coil driven by the above ball release relay is what serves up a ball into the shooter... this is disabled briefly by the make and break out hole switch, when the out hole relay pulls in second ball relay pulls in, if there are two balls resting on switches, this is how the game knows you don't still have a ball in play and you can score your bonus "and fully drain" somewhere else in the schematic there's a second ball relay contact and outhole contact that is going to pull in the bonus score relay if memory serves, and of course once you're scoring bonus there's a back make directly in series with the ball release so it doesn't serve another ball into the shooter while bonus is counting. NOW on my game getting a "free ball" happens occasionally because the outhole kicker fires it up the trough and sometimes it jumbles and ends up finally going over the trough Hill after the score motor has stopped running, and ball release pulls in and serves up a ball as if the outhole switch was skipped entirely