Minecraft wiki entry for Trapdoor, associated text: A trapdoor can be used as a switchable barrier to entity movement. Although primarily used to block movement by mobs and players, a trapdoor can also be used to control the movement of boats (for example, a top trapdoor placed in a two-wide water flow stops a boat when closed (extended out into the water flow), but allow it to move again when open), items and minecarts (a trapdoor can stop a falling item or minecart, then allow it to drop again when the trapdoor opens), etc.