Minecraft wiki entry for Transmission, associated text: Behavior (falling-edge): When the input turns off, the upper sticky piston begins to retract the block of redstone, immediately cutting off power to the output, effectively allowing the falling edge to pass through the circuit with no delay. While the block of redstone is moving, the lower sticky piston deactivates, but then activates again when the block of redstone stops moving and can activate the lower sticky piston by connectivity again.