Revvi 18/20 Handlebar Lanyard Cut Off Switch.
This is a genuine Revvi part for the 18" and 20" models.
A handlebar mounted switch that can be plugged into one of the brake lever cut off wires from the control unit.
The rider attaches the lanyard to their wrist. If the rider falls off the bike then it will remove the magnet from the switch and cut off the bikes power.