It’s simple. If the spindle in your lock doesn’t turn, the door won’t open. Lock-lock grabs the spindle so the handle will not open the door even if the lock is broken.

All you need to do from the inside is either lift the handle and flick the switch to lock it, or, if you have installed the key version, use the key. It is perfect for any door you don’t lock from the outside like conservatories and French doors.