softshot:
Mike Sigel does this as well, even with his index-overlapping-middle finger closed bridge, albeit Mike does this with his ring finger. There could be a couple reasons for this behavior:
1. Ronnie is "reminding" himself to keep the weight of the bridge hand on the outer perimeter of the bridge hand tripod (i.e. the index finger, and the ring/pinky combination). He could be "ensuring" that the tripod formed by his bridge is truly resting on its outermost "legs" (index + ring/pinky) to spread the weight out for maximum stability. I find myself doing this from time to time, and like you, I asked "why do I do that?" Then, when I caught myself doing it one day, I discovered that I was unconsciously "ensuring" the tripod was made up of its outermost legs (spreading the bridge's contact with the table out) to ensure stability. It's actually a great thing to add to your pre-shot routine.
2. Ronnie's middle finger is his "shot divining rod" -- as he's down on the shot and going through his practice strokes, making little adjustments, the middle finger waggles as sort of a "ding-ding-ding -- yeah, that's it, that's the line of aim!"
Ok, ok, I was kidding on that second one.
Hope this helps!
-Sean
I also find myself doing this sometimes and it is not intentional.
It is just a result of the way snooker players form their bridge. The classic way they are taught is to spread the fingers of the bridge hand as wide as you can and then really grip the cloth with the finger tips. Then move the thumb in to make a V
If you try doing this (on any flat surface) you should find that the finger tips of 3 of the 4 fingers are tight against the bed: If you lift any of these 3 fingers up with your other hand, you will find that it is tense - it snaps back when released. But with the 4th finger is loose - it lifts up quite easily; and sometimes it lifts up off the bed of its own accord.
This combination of 3 tense fingers and one loose one often results in the characteristic bobbing. If you can find an old video of Tony Meo playing snooker you will see it in spades.