It don't mean a thing if it ain't got that ping.
The key is that if you feel that ring finger in the lead, you're holding the cue consistently. It's an easy non-thinking feel checklist. It applies to all shots. For some reason the ring finger feels right (maybe because it's dumb and not trying to take control like the more dexterous fingers). There's not magic in it, but if you build it into your psr it's another thing you can offload. It only feels correct if you're holding the cue well and your mechanics are good. Do I feel it? Yes... ok ready to perform.I don't want to sound like a pedant, but for pool as opposed to snooker, the hand does not need to butterfly open then closed again, as many others have remarked.
And--please take this in the kind spirit in which it's given--there are at least two reasons for focusing on, say, the index finger, one is mechanics and consistency as clearly explained above by many, but the other is to create a checkoff point or feel point to ease nerves. I don't teach a finger point because I teach a relaxed body and mind, if you follow (or draw upon, haha, I said "draw" and "follow") what I'm writing here.
Put differently, the culprit for players who are having a tough time is usually tip gap or stroke motion or aim system and I've not ever told a player, "love it but get more action with the index and less with the pinky". I'm OPEN to what a great snooker coach has to share, but just sayin'. And that includes all levels I've coached from rank beginner on up.