The coin lockout mechanism is part of the coin acceptor, it isn't specific to MVS cabs.
- When powered, it allows an inserted coin to hit the coin switch and go in the coin box.
- When not powered, it defaults to a "safe" position which makes the coin take the return path.
This mechanism and the watchdog are failsafe measures to prevent users from wasting an inserted coin when the cab isn't operating (properly).
Coin lockouts are typically coils hardwired to the 12V power supply. It's the game PCB's job to power them by grounding their second terminal to close the circuit. See JAMMA connector pinout.