For future Pinside searchers: I was frustrated not to find a reliable description of the add-a-ball startup sequence (especially in light of the WOW feature, which turned out to be relatively straightforward once I worked through it) and here is how it operates on Bank Shot:

1. Inserting a coin activates the Start relay (S) via the 1-coin-1-play option.*
2. S activates the Tilt Hold relay (H) to power the playfield. (Tilting the machine will deactivate H.)
3. S activates the Hold relay (R) to power the backbox. (Slamming the machine will deactivate R.)
4. S activates the Reset Completed relay (AX) to latch on.
5. S starts the motor turning.
6. S and Motor resets the relay bank, then S deactivates.
7. AX keeps the motor turning.
8. AX and Motor resets the score reels to zero.
9. AX and Motor decrements the Lightbox Advance unit (WOW) to its lowest (zero) position.
10. AX and Motor increments the Ball Count unit (BCU) to its highest position (six).**
11. AX trips off after these resets are completed (via Motor, BCU position-six, WOW position-zero, and all score reels position-zero), restoring power to the outhole and stopping the motor.
12. The ball resting in the outhole has closed the Ball Return switch, which activates the Bonus Score relay (Q).
13. Q activates the Motor so the bonus unit will increment to its highest position (fifteen), activate the Ball Return relay (O) via a stepper switch at fifteen, and then reset to the zero position.
14. O kicks the ball out to the shooter lane, closing the Trough switch along the way.
15. The Trough switch decrements the Ball Count unit from position six to position five, lighting Ball "5” in the backglass. The game is now ready to play.

*If set for 1-coin-2-plays, the coin switch instead activates the Two Play relay (X) which activates S. Note it also activates Credit Hold (Y) which will hold itself on, lighting the apron and enabling the Start button. Therefore, if Y is energized, pressing the Start button can also activate the S relay. (Based on this, I chose to set free play by bending one switch in the Y stack so that the Start button works but the Y relay is not continuously on.)

**If set for three-ball, the relay CX will now activate, and the unit will decrement two steps before deactivating CX in order to start you at 3 balls.

*** Every time the ball drains to the outhole, you can expect steps 12 to 15 to repeat.

This is an oversimplification in order to highlight the critical path in Bank Shot & maybe someday I’ll document the motor switch IDs.