Minecraft wiki entry for Telegraph, associated text: In addition to being able to loop, redstone repeaters have the advantage of being able to be "locked" to a state. Thus, if a message is sent in a pre-configured pulse, it's possible to have a pulse preceding the message to lock the message in place in the repeaters, removing the need for a loop, and adding the possibility of a "timed eraser" if executed correctly. To lock a repeater in its current state, a repeater must power the side of it, then locking the current state.