http://breezewiki.catsarchywsyuss6jdxlypsw5dc7owd5u5tr6bujxb7o6xw2hipqehyd.onion/minecraft/wiki/Observer
It also counts as a block update when the observer itself is moved by a piston. When this happens, an observer emits a pulse after being pushed or pulled, but not beforehand. This enables them to be used in flying machines . Observers behave as both opaque and transparent blocks: they block light and allow mob spawning on top, but do not block opening chests below, do not cut off redstone wire, and cannot conduct redstone power.