Minecraft wiki entry for Transmission, associated text: When an input turns on, it (a) turns off the torch on the side of the block and (b) powers the block in front of the input, activating the sticky piston on the other side. When the piston starts moving its block, this instantly allows the powered dust underneath to connect to the output. By the time the power from the torch and repeater have turned off, the block has arrived at its extended position where it connects the power from the other torch and repeater to the output.