What makes a "floater" move? or a topspin serve dive?

The "float" serve moves erratically by creating unstable and unpredictable air currents around the ball resulting in unstable "lift" causing the ball to move in mysterious ways. No one, not even the server, knows where it's headed. The faster the "floater," the more it moves as the forces acting on it are greater. And no, it's not due to a certain position (out or up) of the valve at the time of the serve. And no, it's not caused by the bladder and ball expanding/contracting/pulsing after being hit.

A topspin or sidespin ball dives or moves based on unequal pressure on the ball caused by the rotation. The pressure is higher on the surface of the ball moving more quickly in the direction of flight than in on the surface rotating away. The ball appears to be "pushed" away from the surface with higher pressure. A rough surface or prominent seams may exaggerate this effect.