Aerodynamics of Flight Control Surfaces
Thankfully, aerodynamics in the usual flight range is linear.

Therefore, there is a gradient of lift over angle of attack and another one over the flap deflection angle. Both are constant over a range of maybe ±15° and can be combined. The angle of attack is referenced to the fixed part of the flight surface and the deflection angle to the moving part relative to the fixed part. 

Another parameter which influences lift is the camber of the flight surface. Positive camber produces more lift at the same angle of attack. Deflecting a flap changes this camber, and its effect can be linearly added to that of the angle of attack.


  Is it air being deflected thus a change in momentum occurring to pivot the aircraft's tail down?


Yes, when a flap moves, the angle of attack stays constant but the camber of the flight surface changes, thus producing a change in lift. In your example, a trailing-edge up deflection decreases camber, the surface produces less or negative lift which produces a moment around the center of gravity.

And yes, lift is produced by deflecting a stream of air.

Things get more complicated and interesting when the aircraft leaves the angle of attack range in which aerodynamics is linear.