http://en.wikipedia.org/wiki/Tidal_locking "The angular momentum lost due to the slowdown is balanced by a speeding up along the orbit caused by the other body pulling on the bulge. Since it takes a small but nonzero amount of time for the bulge to shift position, it is always located slightly away from the nearest point to the other object. For the case of a rotation period shorter than the orbital period, this bulge is located in the direction of the rotation, pointing slightly backwards to the direction of orbital movement. Because of this misalignment a component of the gravitational attraction by the other body on the bulge is then directed along the direction of orbital motion. This steady but significant pull on the bulge accelerates the first body along its orbit, boosting its orbital angular momentum. In the opposite case of a rotation period longer than the orbital period, the moving bulge drives the body while holding its orbital movement back, and the rate of rotation is increased at the expense of orbital momentum instead."