Ng,
like they were saying it seems like u have the basic idea. actually when you release a bowling ball, (this is the easiest way i can explain it) when you are doing your approach to the foul line, you are in ur back swing, when the ball reaches around you hip, your thumb should already be out of the ball, ur hand turns to the side but staying underneath the ball, and rip thru the ball, mostly with your ring finger giving you all the lift and rotation. i hope this helps, it is not the best explantion. and as for your question about the fit of your thumb, this is another issue. is it too loose or to tight? if it was drilled correctly they can put either forward or reverse pitch into the thumb hole. let me explain this, reverse pitch exactly what it says ur thumb will be basically pointing straight behind you, this is for ppl that tend to hang up in the ball alot, with a normal 0 pitch in the thumb hole. and will allow you to come out of the ball much smoother. forward pitch is the oppossite where your thumb is facing forward, this is done with bowlers that tend to drop the ball upon release and it will give them that extra second with the thumb in the ball.
my balls are all drilled exactly the same. with alittle reverse pitch in my thumb to allow my thumb to come out smoothly. but to answer your question the size of the hole for the thumb should be equivilant to that of a slip fit of 2 fittings.
just remember on your release when you lift thru the ball bring your hand up like you are shaking someones hand and your thumb should be at 12 o clock.
i hope this helps some