[Maurice Johnson]

We know that the bores wear on the thrust face because we can measure the wear with a micrometer.
On the power stroke the ring must therefore be compressed more tightly into the piston groove, peaking friction in line with the conn rod, and decreasing the ring contact pressure in both direction away from the highest pressure point.
If the ring has rotated so that the gap is aligned with the fore and aft centerline of the engine, then that quarter of the ring will exert less force against the bore wall than the other 3/4 of the ring.
Unequal pressure causes the weaker force to give way to the stronger in the form of rotation.
Phew!!-that used up some brain cells-I'm gonna get me a rum and coke.