Minecraft wiki entry for Piston_uses, associated text: Like disappearing floors, but in reverse, hidden bridges show an impassible obstacle (usually lava or a long fall) to hinder other players or kill mobs. These bridges can be activated by lever to allow a player to cross, then deactivated on the other side or by a set delay, done by placing redstone repeaters and setting the second redstone torch to any four positions (Right clicking it). Adding multiple repeaters can make a longer delay if necessary.