Speedway Bike Explained:

CUT OFF SAFETY SWITCH

All speedway bikes must have a kill switch that if triggered while the engine is running, will cut off the electrical circuit and the engine will stop.

This is done through a cord attached to the riders right wrist directly to the kill switch located next to the throttle. The switch is spring loaded and when the cord is pulled away, this closes the signal to the ignition and the engine immediately stops.