Mixing is where you set up two different control surfaces to operate at the same time by only moving one channel on the radio at a time. For example, commonly when you deploy flaps the nose will pitch up. So you "mix" in a little down elevator. If your flaps are deployed fully you may set the elevator to move about 15% downward to keep the nose level. You don't have to move the elevator stick to correct the pitching effect you get from the flaps. Sometimes on the high performance planes when the elevator is yanked back, and the plane does a hard pull upward, a little torque from the engine applies. Yaws, or rolls the plane left. You might mix in a little rudder, or aileron to counteract the effectof that torque. All you do is pull hard bck on the stick, and the mix that you programmed in applies a little aileron or rudder, depending on what you set-up. It's a way to counteract some of the adverse effects on and aircraft when you do something other than striaght and normal flight. You let that $250 radio earn it's keep and give it chance to help you do some of the flyin