Right, lets replace the plane with a car. If the car's wheels were moving at a potential 5mph and the conveyor belt was moving at 5 mph, the car would be effectively stationary. This is because the car's power is directed through the wheels into the conveyer but the conveyoer is directly cancelling out this movement by moving the opposite way. Now, an aircraft moves by directing it's power from the engines, into the surrounding air. Because the power doesn't go into the ground in any way, the plane will move and will take off. You'd have no problem accepting this if the plane wasn't touching the ground but really it's no different when it is touching the ground. What's happening below the engines does not affect the way the engines effect the plane. It's alot clearer in my head......