If you wait by a door to be opened by a guard, then stun him, the door stays open. Usually one can use this passage to the other room and “climb” over the stunned guard. This seems to be impossible in some targets, e.g. Kennwood House. The guard is stunned, the door open, but at most the person who stunned him can pass through the door, no other person.
This is not how I remember it, also it makes targets like the Kenwood House quite impossible.
It doesn’t matter if the guard is attacked before or after his passage through the door. He just blocks it.