That Masterclass bit on the 28th March Ronnie O'Sullivan show was a replay from an earlier episode. I'm fairly certain that during his feathering, his grip is the textbook loose grip. But as he strikes the white, he alters the pressure of the grip depending on the power of the shot.
One common textbook snooker technique is changing the length of the backswing while maintaining the same cue speed throughout the stroke. For most shots, Ronnie prefers using the same backswing length AND cue speed; the only thing that's changing is the pressure of the grip. When you watch him on the telly you hardly see a difference in the pressure grip when he performs power shots, but you know that it's there--he can deepscrew the ball back with what looks like the same pace he uses for softer screw shots. The trick is to apply the pressure grip at the exact moment the cue hits the white. So really, the key to this technique is TIMING, which Ronnie has an extraordinary gift for.
I tried it out quite recently just for the kicks. To convince myself of his technique, I used a liberal form of his grip: as I reached the end of my backswing, I opened up all my fingers away from the cue, which is only held up firmly by the 'V' between my thumb and forefinger. At the moment of strike I then close all my fingers back around the cue to simulate the pressure grip. While I still find it inconsistent, it nonetheless worked wonders on a snooker table--knocking in table-length straight in shots became much easier. The difficult part was figuring out the pressure grip for different paces of shots. Ultimately I didn't adopt Ronnie's technique because he has a different eye rhythm and natural tempo from mine, both of which are crucial for timing.
