Funny you should post in this thread today!
Today I figured out what my mysterious yellow knob is. I also figured out that it isn't an on/off switch either. Turns out the "push on", "pull off" writing on the knob refers to the lock function of the knob. When you pull it out you can rotate it, when you push it in it locks and won't rotate. So what the thing is is an air pressure regulator; once you get it set where you want you push in on the knob and there it stays.
And what air pressure does it regulate you ask (I know I did!)? The door! So I got both answer (about the knob and why the doors won't stay closed) at the same time.
I cranked that knob all the way to the right and now you couldn't open the doors with a crowbar!. I'm still not certain why
the door actuator has a pressure regulator but at least I know it does and the doors don't blow open now.
This regulator is in addition to the valve near the door that lets you bleed air pressure (from the actuator) altogether so that you can come and go by manually opening and closing the doors.