Minecraft wiki entry for Transmission, associated text: Behavior (falling-edge): When the input turns off, the lower sticky piston begins retracting the block of redstone, immediately cutting off power to the output, effectively allowing the falling edge to pass through the circuit with no delay. The block of redstone then arrives at its retracted state and tries to power the output dust again, but it also powers the piston above it and block A arrives to cut the output before the repeater can output the signal from the block of redstone.