I asked what specifically because I used to do my thumbhole and grips like Norm...inspired by what I saw him do.
The fingers are sharp to keep you from grabbing at it...if you pinch it at the bottom...you get immediate feedback. If you bowl a block where you keep grabbing it...your fingers will be very sore the next day.
As far as the thumb...it's a way of getting the feedback of a sharp hole, but w/o the penalty of hanging up in it if you happen to grab one. You take a rasp and work out the to of the hole, keeping the edge sharp but creating some bevel so the thumb can clear it. This was a great way to help you stay relaxed in the ball, but not embarass yourself when you didn't throw one pure.
Little known fact: In Russian "Hope" and "Change" translate to "Tax" and "Spend"