What is the meaning of "tidal coupling"?
Tidal coupling or synchronous rotation is what causes one astronomical object's face to be fixed pointing to another, just as the Moon's visible face is always pointing to the Earth. A coupled object takes the same amount of time to rotate about its axis as it does to translate around its companion. This synchronous rotation, also known as captured rotation or synchronous rotation, causes one hemisphere to point continuously toward the companion object. Normally, only the satellite couples around a larger planet, but if the mass difference between the two bodies and the distance between them is small, both objects may have a reciprocal tidal coupling, as Pluto and Charon do.