Minecraft wiki entry for Redstone_components, associated text: A tripwire hook turns ON when an entity (mob, item, etc.) crosses or falls on the hook's tripwire line (but not the tripwire hook), and turns OFF when all entities leave or are removed from the tripwire line. A tripwire hook also turns ON for 5 ticks (1/2 second) when any of its tripwires are destroyed, except when using shears to cut the tripwire. Breaking the tripwire hook, or its attachment block, does not generate a pulse.