You are talking about the belt moving in the opposite direction of the plane, right?

Think of a car on an oversized treadmill. If you set the treadmill to 25 mph, and set the cruise control on your car to 25 mph, then youŽre not going to move anywhere (air speed will equal 0 with no wind). If you hit the gas and drive 30 mph then youŽll move forward and your air speed will be 5 mph, and the wheels will be turning 5 mph faster than the belt.

So the only time that the wheel speed and belt speed are equal is when the air speed is zero. When the thrust of the car is applied to the wheels the belt can always keep up and the airspeed will remain at zero. But when you switch on the JATO rocket attached to the back bumper of your car, then the thrust is applied to the air which of course makes the airspeed (and earth-ground-speed) increase, forcing the wheels to turn faster than the belt. If the belt tries to keep up by spinning faster, then itŽll only further increase the speed of the free-rolling wheels.