3 Way Switch Only Works When One Switch Is On . How it works is that the hot wire coming in from the electrical panel is connected to the common screw of the first light switch in the circuit. Wired up a 3 way switch. ThreeWay Switches & How They Work Instructables from www.instructables.com The wires that connect the two light switches to each other) attach to these two screws, and it doesn’t matter. The hot in this schematic is never connected to the lamp unless the lamp is illuminated (unlike in a carter). #2 when removing the wires from the old switch not taking note of what wires are going to what screws.