On cylinder rim locks with this function, once you have left the house and closed the door, insert the key in the cylinder and give it an anti-clockwise turn. This deadlocks the bolt preventing it from being pushed back.

This facility in a rim lock which, using an extra key turn on the outside, stops the latch or bolt from operating when the door is.

facility in a rim lock which, using an extra turn of the key on the outside, prevents the latch or bolt from being operated when the door is closed (see also Automatic Deadlocking).