Minecraft wiki entry for Transmission, associated text: Some transparent blocks can support redstone dust: hoppers, glowstone, upside-down slabs, and upside-down stairs. These blocks have the property that redstone dust on them can propagate signals diagonally upward, but not diagonally downward (transparent blocks that cannot support redstone dust cannot be used for this purpose). Thus, simply jumping the signal up one block to one of these transparent blocks creates a diode circuit.