The CD-ANA-2 board is the audio, video and power board for the front-loading NeoGeo CD. It was later modified and named NEO-CDA in top-loading systems.
MONO goes to the video encoder for amplification and output to the DIN connector.
The circuit is a fairly complex toggle switch with power-on reset and debouncing.
POWER is constantly at 5V when the power plug is connected.
VCC goes to 5V when STARTUP goes low.