Given that differential forces will stretch out a dimensional body along the axis of gravitational attraction we can refer to this stretched out portion as a “bulge”. For masses such as the moon, which always shows us the same side rather constantly, this bulge is fixed and creates a slight elongation, however, for rotating objects like the Earth, this bulge moves around the surface always moving to point toward the source of the gravitational attraction.

     The bulge closer to the attracting mass M is attracted more strongly by the gravity of mass M than the more distant bulge. Thus this rotating bulge creates a small torque as the bulge, constantly displaced by the planet’s rotation (the planet not being perfectly elastic), is pulled in to align with the axis of gravitational attraction.

     This small torque acts to slow down or speed up the rotation of the mass of radius R until its period of revolution catches up to its tides or, in other words, the mass of radius R is affected to bring its periods of revolution in synchronization with the orbital period of mass M.

     Thus differential forces serve to ultimately bring about a situation in which mass M always sees the same face of the mass of radius R.

     This, in fact, is what has already occurred with the Earth’s moon which now revolves on its axis in synchronization with its revolution about the Earth, always showing the same face. The Earth, likewise, which revolves on its axis faster than the moon is slowed down in its rotation by differential forces so that the Earth will eventually always show the same face towards the moon. This slow-down is currently of the order of about .0016 seconds per century. (Modern Astrophysics, p. 764)

     For an example of where this process has already fully played itself out one can look to Pluto and its moon Charon where both bodies show the same face to each other constantly. (Dynamics, Todd J. Henry)