Minecraft wiki entry for Tripwire_Hook, associated text: While active, a tripwire hook appears to move downward and it:
powers any adjacent redstone dust to power level 15, including beneath the tripwire hook
powers any adjacent redstone comparators or redstone repeaters facing away from the tripwire hook to power level 15
strongly powers its attachment block to power level 15
activates any adjacent mechanism components, including above or below, such as pistons, redstone lamps, etc.