Open door locks with spring latchbolts. Just shove the device behind the latchbolt and push down and forward to force the bolt back into the lock housing. Also works on certain deadlatching locks if you can use a wedge to flex the door jamb so that the deadlatch or anti-loiding pin extends, allowing the latchbolt to be retracted. Not for locks with deadbolts or deadlatches that can't be wedged from the jamb.
