Flipping the switch won’t unlock the D-2 extraction point until you press a separate button, but may spawn raiders in that underground area. In the underground area beneath the Pawn, Bishop, and King buildings, is another switch in a breaker room. The alarm is loud enough to hear from almost every area on the map, so you’ll know if someone has flipped the switch, and to then be cautious in the Train Station area. When the switch is flipped, Raiders have a chance to spawn a number of Raiders near the extraction point itself, in the vicinity of the Train Station on the eastern side of the map. Flipping the switch on the inside will cause an alarm to sound, and a timer for the Bunker Hermetic Door extraction to begin. Near the western side of the map, next to the building marked with a White Pawn, is a small breaker room. Unlike the Lab, Reserve doesn’t only spawn raiders as normal enemies, but you can perform certain actions to open extraction points and spawn raiders.
Occasionally, raiders will enter from behind the previously closed gate, so be careful of standing in front of it, and if you intend to use the gate as an extraction as well.
As soon as you press either of these buttons, you may want to vacate the room and find a defensible position, as Raiders will be spawned and will head towards the gate.