Minecraft wiki entry for Leaves, associated text: The state of a leaves block—including a player-placed block—changes after 1 game tick (half a redstone tick) when the distance to the nearest log or wood block changes, up to 6 blocks of leaves away. Observers facing away from the leaves detect this change and transmit a redstone signal in the same game tick, making leaves useful for redstone signal transmission. This has been called "Leafstone" by the Minecraft Community.