thyristor (SCR)

A latching power switch, like a mousetrap: a tiny pulse on the gate snaps it closed, and it stays closed even after you let go. Once a thyristor (silicon controlled rectifier) is triggered, current flows until the current itself drops to near zero — which conveniently happens every half-cycle in an AC line, resetting it automatically.

Because it self-resets at the AC zero crossing, you control power by choosing when in each cycle to fire it: fire early for more power, late for less. This 'phase control' runs dimmers, soft-starters, and giant HVDC converter stations handling thousands of amps. The downside is it can't be turned off on command — for that, designers reach for IGBTs.