Minecraft wiki entry for Redstone_components, associated text: A pressure plate is not solid (it cannot be used as a wall or platform). Usually a block under a pressure plate provides solid ground (for mobs to walk across, items to fall on, etc.), but when a pressure plate is placed on a block with a small collision mask, like a fence or nether brick fence, it is possible for entities to move through the pressure plate and still activate it. Thus, a pressure plate on a fence can be used to detect entities without stopping them (more compactly than a tripwire circuit).