It's easier to think of them as sealing. The motoworks url above gives a good explanation. The ring has to seal between the ring and the land, and between the edge of the ring and the bore. It seals or seats by coming in very very close to the surfaces it's sealing against. There should be a very thin film of oil between all three surfaces.
There is an unbalanced radial force from the cylinder pressure that loads the ring against the cylinder wall. Think about it. Pressure from the cylinder pushes outwards on the ring while once it is in very very close contact with the cylinder wall there is no pressure pushing it back. I believe the ring is "seated" when the cylinder wall and ring outside edge has worn to let them exclude pressure from the cylinder.
The force of the gas on the ring pushes it against the ring land to seal the sides.

In modern times piston manufacturers like Mahle have found that making the ring lands extremely flat, like within 6 millionths of an inch, increases power by giving a better seal. The radial edges are designed to wear in quickly. The rings also twist when in service which is probably why a new compression ring has very sharp corners, those would be the intended wear points.

The only rings that do not move on a piston are either stuck or there is a pin stuck in the ring slot by a designer. Long ago I read that certain chevy race engines of the early '60s had a problem with ring wear because the rings would spin too rapidly. But rings, like valves, will rotate by themselves.

Keep in mind that the rings are wearing at the cylinder walls when the walls are loaded by bolts or studs and hot. I believe they actually make the cylinder more round in its true operating condition.

R