I am by no means a whiz, so somebody please correct me if I'm completely off the mark, but the way I understand it is like this: rotatation at the poles is 'tighter' (smaller rotations = more force generated), and 'lazier' at the equator, hense less force. Sorta like the inside and outside of a turn.
I'd love to hear from a higher authority on this one!
cheers!