Minecraft wiki entry for Pressure_Plate, associated text: A pressure plate is not solid (it is not a barrier to entity movement). A block under a pressure plate can provide a solid barrier underneath it (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, such as a fence or nether brick fence, it is possible for entities to move through the pressure plate while still activating it (walking through it horizontally, or falling through it vertically). Thus, a pressure plate on a fence can be used to detect entities without stopping them (more compactly than a tripwire circuit).