Years ago, I had taken a couple of lessons from John Jowdy. John was very big on
getting bowlers into the habit of creating that flat spot with the hand at release.
He said that he taught this even back in the days of the rubber/plastic ball era. He said that the great Earl Anthony was one of the few top pros in those days that purposely
worked on creating this motion for himself. In fact, Earl once commented that when he released the ball he would try to imagine that his bowling arm could unfold (telescopically) so that when the ball got to the breakpoint, he tried to envision his (bowling) hand still
on the ball (and not above it). He referred to it as "chasing the ball downlane with the hand". This prevented him from hitting up on the ball.
However hitting "up" and squeezing are 2 different things. Squeezing at release was (and is) usually very counterproductive to getting a consistent release and maximizing carry. However, releasing the ball on a slight upswing is usually
beneficial for bowlers who still deploy the older style release----- where the bowling hand
releases more from the side of the ball (versus from under it). By releasing it on the downswing (where the thumb exits next to the slide ankle), the ball's rotational axis
tends more towards the vertical . This will usually cause such a release to be ineffective, versus releasing it on the slight upswing. By releasing it in this fashion,
the ball will have axis tilt, but not to the extreme degree as is is when released on the downswing, at the ankle. This results in better pin carry for these "side-ballers".
This type of release is still common among bowlers who learned the game 40+ years ago. When I instruct bowlers in this category, I often find that the easiest way to get
them to release on the upswing is by keeping the shoulders behind the slide knee to a degree. Too much forward shoulder lean makes it much more difficult to accomplish
without the bowler wanting to squeeze the ball at release.
For the more contemporary release (where the wrist collapses), it is usually more advantageous to do precisely the opposite, more forward shoulder lean allows such bowlers to more easily move their finger through a larger arc (circumference) of the ball at release with less axis tilt.