Minecraft wiki entry for Redstone_components, associated text: In addition to the methods above, pistons, dispensers, and droppers can also be turned ON if a block above it receives a block update (including a redstone update within two blocks of the component) and is powered by any of the above means, even without a mechanism component (e.g.; even if the block above the component is air or a transparent block) . This rule is often simplified to say that the components can be powered by blocks diagonally above or two blocks above, however other methods of activation by connectivity exist (see image to the right). This method of activation is also known as "connectivity", "piston connectivity" (as it originated with pistons), or simply "indirect power".