You should have left this question, for the next person you're bowling against, until, say, they are about to bowl the 9th. frame.
It's what those of us with backswings do...Thumb in the ball? What's that?
Give it a go. Thumb in first reduces grip pressure in the fingers, as you can't dig them in as hard) and therefore reduces lift and loft in my experience. Makes the ball go faster through the front. Must add that to the "try at league" list for tonight...Fingers 1st then thumb doubt if it makes a difference
As far as I know though, you need a slightly under-spanned drilling to be able to get the fingers in enough to put the thumb in first.
It must work for some, I mean look at Tommy Jones, he does it and it works for him, he says the way he does it (like Bluey said, thumb in, 90 degree turn, insert fingers) helps him get this thumb out clean.
I'm sure Billy Gardiner puts the thumb in first.
willey