Minecraft wiki entry for Mechanisms, associated text: This is intended to be used in narrow passageways. Since a piston takes 1 tick to extend, you can make a tiny monostable circuit that is activated by a pressure plate. Then, place a sticky piston with a block attached one block away from the monostable circuit, in which the sticky piston is connected to another pressure plate, one block away from the other. Then connect the blocks on the bottom layers with repeaters. The output is the block that is pushed by the sticky piston. The output is on when a player walks from one pressure plate to the other, but only in one direction. In the diagram, it looks like this (the inputs are below pressure plates):