I tell all my students from the start of the swing until you release the ball to imagine you’re holding an egg. If you squeeze the egg you have damage the egg and your shot.

What do you tell them when the ball slips when they don't squeeze the snot out of it? tape? What do you do when they have so much tape in the hole that they have to twist their thumb into the hole and it still falls off their hand? You can't out coach a bad fit, so start your lessons with that since 90% of bowlers don't have a ball that will not stay on their hands without squeezing it.

I agree 100%. Checking the fit is the 1st thing we do. We have a very good ball driller at the bowling alley. If they don’t want to get the ball check then I tell them you are limiting how good you can get.
This usually happens with older bowlers rolling old equipment (8-10 years old) and bowlers getting their own ball for the first time. A bad fit may feel good because that’s what your use to. Some bowlers don’t what to change their span or pitches and they will never know what it like is to roll a “clean” pure shot. Bad for them.

I agree.. if you squeeze the ball, more than likely you're doing it without even thinking about it. You will need to consciously think about not squeezing it the entire approach. Think about your thumb being straight, hand extremely relaxed, fingers relaxed and keeping it that way for the entire delivery.

Lately I've been noticing my hand tightening up so I've made it a point to add a step to my routine to make sure my hand is relaxed.

Lefty, you might want to move your pitch forward 1/8", that is what happens when your hand has learned to relax past the point of where the thumb pitch is, your hand starts to tighten up to prevent the ball from slipping from your thumb. Try going another eighth forward and see if that helps your hand relax more without thinking about it.

Okay, So I think I'm finally going to change my pitch. How much to start out with? Forward 1/8"?

I have grown to trust not dropping the ball thereby not bending my thumb to hold the ball, however I find it difficult to keep my hand extremely relaxed and cup my wrist at the same time. When I cup my wrist, both my hand and forearm tense up under the weight of the ball to maintain the cupped position throughout my swing.

Why is your bowling hand supporting the weight of the ball? The only time your bowling hand should have any weight on it is at the bottom of the swing. If it does at any other point, you are controlling the swing.

Relax your hand, then cup your wrist. Make sure you can wiggle your fingers freely. Now push on your palm with your other hand. Wiggle your fingers freely. You can keep a cupped wrist with a relaxed hand.