The rate of spin of each ball remains constant, but since its "orbital speed" varies with its distance from the centerpoint of the rotor's circle--not the distance from the centerpoint of the circle inscribed by the balls--the actual speed of each ball varies. They are going slowest at TDC and fastest at BDC.

The track causes the ball to speed up (as the track comes together) and slow down (as the track separates) in exact proportion to the amount of speed change required to keep the ball in radial alignment within the cylinder. Ergo: Extremely low friction!

This pump is in the experimental stage as this is being written. This design and several similar ones are also being developed for use as engines and compressors.