more generically, features almost always enable by either a stepper unit reaching a certain step or a trip relay tripping. The easiest way to tell is look on the schem in the upper area where the lamps are and see what turns on the lamp. Ignore the circuit coming from the spotting disc that is just flashing the lamps during the cycle.
I think MrBally meant the reflex unit. When the game pays out, the reflex wipers step up slowly and disconnect circuits. That makes the game more stingy, so the ideal thing is to play for extra balls before collecting wins, especially big wins.
the EB adjust plug should be set to connect as many wires as possible between the plug and socket, and as usual the game uses "proportioning" to make getting extra balls harder if the scores are high or more features are enabled. Test extra ball functionality with a 1 coin/credit game ... you should get the extra balls pretty quickly.
if you don't like how infrequently the game is giving extra balls when scores/features are better, then first check the EB adjust plug then see what the reflex unit is doing. If reflex is reset and the plug is most liberal, you'll need to check circuits to make sure the operator didn't cut wires on the spotting disc or mixers.
it's possible to calculate the odds of getting an extra ball if you have the spotting disc diagram from the manual, mixer and reflex diagrams from bingo.cdyn.com, and the patience to work thru all the circuits to see how stepper unit positions, trip relay states and alternator cam switches enable/disable various circuit paths.