Minecraft wiki entry for Transmission, associated text: Scaffolding provides the upward counterpart: Each scaffolding block tracks how far it is "overhanging", that is the distance from a column of scaffolding which is actually resting on a block. A trapdoor counts as support only when it is closed. Place a column of scaffolding atop a trapdoor, and also next to another scaffolding block that can support it while the trapdoor is open. When the trapdoor is closed, the column isn't overhanging at all, when the trapdoor is open it is overhanging 1 more space than its support scaffold. The difference propagates up the column, and can be detected by an observer.