This is a Surefire Remote Switch known as the DG Grip Switch, model DG-11.
For surgical control when using an X-Series Weapon Light (X300" X400)" the DG grip switch uses a minimal center-mounted pad. This allows the operator to activate the light with finger pressure without needing to move a finger from the grip.
Features:
Ergonomic Design
Ambidextrous dual momentary and constant-on push/toggle switch
Remote switch uses a minimal center-mounted pad