This flap remains leak-proof while the pressurization works (resisting up to approximately 100 Pa as standard, or other pressures on demand). It opens quickly when the doors close, letting the pressure escape, allowing you to easily close the doors.
A fully-encapsulated seal guarantees airtightness around the frame of the cab.
Since it is small, it can always be found a spot. Fit them in pairs in the cab, to assure easy door closure.