As zacaj said. Basically, you know that AC "alternating current" means the flow of electrons reverses, and when it crosses zero there is a signal created that informs the MPU that the next cycle has started. Think of it as a clock signal based off the frequency of the alternating current supplied to the game from the AC main. It can be used to refresh the SCRs that hold the lamps on, signal when the best time to discharge coils to avoid spikes, etc.. depending on what the design is.