Not energized immediately after start up, so that the credit button will either add on more players or do nothing. Once ZB is activated, the credit button initiates a reset.

M and N switches cause ZB to activate when the two lowest scoring increments occur (1 and 10 for Groovy). The H switch causes ZB to activate if the first ball is tilted even before any score occurred.

The R switch is there so that if power drops off and is then restored, the credit button initiates a reset. Predecessor games Mini-Cycle and Polo did not have this important switch!
.................David Marston

I had a Crescendo, the 2-player parallel to Groovy. Increased scoring of bumpers and lanes is stepped up based on which ball is in play. (Weird I know, but I confirmed that this was a deliberate design decision in that era.) For a 3-ball game, they would step ahead, so to speak, in the advancement of the score values.
.................David Marston