Minecraft wiki entry for Basic_Piston_CSG, associated text: Design B sends its cobblestone upwards. Rather than a clock, the piston is triggered by actually sensing when the cobblestone is created. The piston is sticky, and holds onto a "switch block" which must be pushable but transparent — that is, can push the cobblestone, but not carry redstone current. Top slabs, etc., will work as well as glass. It is demonstrated in this video.