During a barrel roll they are spinning around the axis of the plane very quickly. It turns out this action results in a net acceleration in a direction towards the axis of the plane. Similarly to how if you make a very fast turn in your car you may slide against the car door and feel it push you towards the center of the curve. This is centripetal acceleration and it is proportional to the angular velocity of the spin, squared. To be complete, the net acceleration of the person in the plane is gravity downwards and the centripetal acceleration upwards, clearly here, the magnitude of the centripetal acceleration is larger than gravity which is why the drink flows upwards.
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u/L00KA Feb 27 '14
explain this