That really was a poor screen shot to show! As mkc says, your hands rest lightly on the knobs. And the ring does such a good job of keeping the quilt flat, I've never had to smooth it out with my hands.
Synnove, I've done large quilts with the ring. The limitation is that you have to move the ring often. I think this must be why they invented the two separate grippers model, so you could quilt a longer line if you wanted to.