Downward movement of the pressure plate moves down a pressure plunger, which in turn levers up the striker retaining lever, releasing the spring-loaded striker and allowing it to impact the stab sensitive detonator, triggering the main charge.
Decrease in pressure plate movement activates a plunger, which subsequently lifts the lever holding the spring-loaded striker, freeing it to strike the detonator sensitive to impact, triggering the main explosive charge.