Minecraft wiki entry for Quasi-connectivity, associated text: This means that when these redstone components activate the space above a piston (one block away), they also simultaneously update the piston as well (two blocks away). Redstone comparators and repeaters can only activate mechanism components adjacent to themselves horizontally, but redstone dust and torches can also activate mechanism components below themselves as well (redstone torches can also activate above, but that doesn't help for QC activation).