What the under-playfield Tilt "T" relay does
The Tilt relay under the playfield pulls in on a tilt: it lights the backbox tilt lamp, turns off the playfield GI and removes power from every playfield coil, so the ball drains. The CPU releases it when the ball reaches the outhole switch.
Systems
Related tips
Source: Repairing & Upgrading Gottlieb System 80 Pinballs from 1980 to 1989, Part One, Clay Harrell (pinrepair.com)