Minecraft wiki entry for Transmission, associated text: Behavior (rising-edge): While the input is off, the block of redstone keeps the lower sticky piston activated by connectivity. When the input turns on, the upper sticky piston begins to extend the block of redstone. The instant the block of redstone starts moving, the lower sticky piston deactivates and begins to retract block A, the reason the upper piston is extending — this turns the upper sticky piston's extension into a 0-tick extension/retraction (the "insta-drop": the sticky piston "drops" its grip on the block and leaves it behind when it retracts), leaving the block of redstone above the lower sticky piston and powering the output. All of this happens instantly (in the same redstone tick), effectively allowing a rising edge to pass through the circuit with no delay. Now that the block of redstone is above the lower sticky piston, the lower sticky piston extends again, and two ticks later block A is back in position causing the upper sticky piston to extend again, ready to retract block A when the signal turns off.