You are under negative g when the aeroplane is upside down and flying level (also when it's right side up, if you dive very steeply, but only when you begin to dive). Being under negative g means gravity, instead of pushing you into your seat, pulls you out of it. To anyone watching (like the pilot in the back seat) someone under negative g - who has undone his straps to get at his camera in this instance - will seem to grow as more and more of his body emerges. If the pilot is quick witted, all he has to do is pull back on the stick - this reverses the g.
If you loop an aeroplane, you can observe the opposite effect - the passenger 'shrinking' under four or five times the usual pull of gravity.
Of course the punishment for all this is when the passenger in the front cockpit is sick. Think about it.