Door One goes from the street to a vestibule, also called foyer, entrace hall, or lobby. Many NYC apartment buildings have this room, Door One is normally unlocked. Residents can enter the space, and so can the mail man to access the mailboxes. Once inside the vestibule, a resident presents a keycard or number code to open Door Two, which leads into the actual building.

While any one door is open, any other door is locked in order to avoid free flow, like in other mantrap scenarios. Also, there is a push-to-lock button inside the vestibule. When pressed, both doors would be locked. This allows a resident to enter the building through Door One and press the button to avoid anyone else following them into the vestibule. Only when they go through Door Two and that closes again, the system resets.