Servos only drain power when they are trying to hold the position they have been told to hold.

The control stick and the trim slide together tell the servo where to go to. The servo will move to that position. Anything that tries to move the servo from that position will cause the servo to try harder to keep itself in position.

Trying to think of an analogy... how about the cruise control on your car.

You set it to 50, the car travels at 50. On the flat sections, the car has to fight air resistance which is trying to slow the car down, so the car automatically "presses the gas pedal" to keep the speed constant. This "draws energy". The car hits an up-hill, the gas gets pressed harder to keep speed. This draws even more energy. The car goes down hill, and the slope counter-acts the air resistance so the pedal is released.... draws less energy even though it is keeping speed. Hits a really steep down-hill, you have to press the brake, consuming energy...

Change the cruise control setting, and the car needs to use energy to adjust to the new setting.

Basically, the position of the stick, and trim, sets the "cruise speed" of the servo, and the energy required to keep the servo in that position dictates the amount of power the servo consumes from the battery. Strong forces on the control surface, regardless of the servo position, will require large amounts of enery to counter-act. No forces on the control surface will require no energy to counter-act.

gus