It occurs in general because, over an accelerating path, the orientation of the velocity varies harmonically relative to that in an inertial frame.
The factor of one-half resulting from this effect cancels with the factor of two error in the naive gyromagnetic ratio, so that a semi-classical calculation of spin-orbit coupling coincidentally yields the same results as the full relativistic one. It occurs in general because, over an accelerating path, the orientation of the velocity varies harmonically relative to that in an inertial frame. Because Lorentz boosts do not commute with rotations, there exists a time dilation between the frame of an orbiting charge and that of a nucleus, so the electron undergoes this motion as it orbits. FTP, name this type of purely relativistic precession most notably undergone by atomic electrons and named for an English physicist.
The factor of one-half resulting from this effect cancels with the factor of two error in the naive gyromagnetic ratio, so that a semi-classical calculation of spin-orbit coupling coincidentally yields the same results as the full relativistic one.
Thomas precession