A signal line. When the console is turned on, it pulls this line high (usually to 5V) to tell the PSU to activate the main 12V rail.
power supply unit (PSU). While Microsoft does not release official internal board-level schematics, reverse-engineered data from enthusiasts and the Xbox Support community provides a comprehensive look at how these units function. Schematics For Xbox 360 Power Supply
This report outlines the technical schematics, pinouts, and operational theory of the A signal line
Provides the high-current power needed for the console's GPU, CPU, and drives. Schematics For Xbox 360 Power Supply
A low-current "always-on" rail that powers the console's power button and wireless receivers while in standby mode.