Hello MTBowler:
For me, the keys for developing a gravity-driven swing rests on the following:
1. The grip
It is very difficult to have a free swing unless you feel secure with your grip.
My thumb pitches and span had to be modified to allow me to swing the
ball with minimal grip pressure. Too much "squeeze" causes the muscles
in your bowling arm to tighten, hindering a free flowing swing. Even with
the proper pitches and span, it is necessary for me to use bowlers tape
at times to accommodate changes in thumb size. A proper fit is important
whether your are an old school stroker (like myself) or you are a contemporary
power player, although it is more important for strokers, who are more on the side
of the ball and for those who utilize "early turn". The amount of thumb bevel also plays
a big role. Too much bevel will cause the ball to fall off the hand too early. The
amount of bevel, pitches and span will differ from bowler to bowler of course.
2. Stance:
Developing a free swing starts here for me. I always make sure to place
the weight of the ball in my opposite hand (left hand) in the stance. I want
the muscles in my bowling (right arm) to be free of any tension.
3. Pushaway
On my pushaway step, I "push" the ball outward with my left hand. My bowling
arm simply goes for the ride. There is no "push" with my bowling arm.
4. Backswing
As I begin the ball's descent into the backswing, I concentrate on keeping
the muscles in my bowling shoulder completely relaxed while maintaining
minimal grip pressure with the ball. In my opinion, this is the point in the
approach where it is most easiest to deploy unwanted grip pressure. This
is another reason why a proper ball fit is important. From this point until
the point of release, I want to keep my bowling shoulder as relaxed as
possible.
5. Downswing and Release
As I begin my slide, I focus on just letting the ball descend up to the release point
on its own---by gravity. This is critical, and this is also the point where a lot
of bowlers will try to "rush" the delivery by using muscle to pull the ball down
from the top of the backswing. Again, this is important especially for those
bowlers who "stroke" the ball and who tend to be on the side of the ball.
Power players usually have a little more leeway and can often get by with
early (downswing) acceleration but even for them, letting gravity do the work
is usually the best way to repeat good shots.
6. Release
Once the ball has descended to the point just before my slide ankle, the thumb
exits the ball. If you have a proper ball fit and if you do not accelerate the ball
on the downswing, the thumb will more readily come out of the ball on its own.
At the point where my thumb exits the ball, I will lightly accelerate the ball with my
fingers directed towards the breakpoint My follow through actually ends up being
higher simply due to the natural momentum created by the gravity-driven
downswing and the light hand acceleration at release.
You also mentioned your head movement and that you prematurely turn the ball.
Without actually seeing you bowl, there can be many reasons for these things to occur. Excessive head movement can be the result of your step gait. Taking too
large of steps can cause excessive head movement. Excessively varying your forward and lateral spine tilts can also be a cause. Too big of a pushaway (or too little) can be another reason for excessive head movement. Many times it is caused by
the bowler "leading the ball" in a downward motion with their shoulders as
the ball is descending into the backswing (the step following the pushaway step) .
If you use a traditional pushaway as I do, it is imperative that as you begin your
swing, the shoulders should not drop forward as the ball descends towards the floor.
As you proceed past this point, your shoulders will (or should) naturally tilt more.
This is not as critical for those who use a hinge-type of release, who often tilt
forward at this point in order to generate a backswing by actually using some muscle to "pull" the ball up .
As for your early turn, this too can be the result of many things too numerous to mention. It can be caused by rolling the elbow out, trying to "work" the ball in order
to create a desired motion, improper foot-to-swing alignment, bad fit, etc etc.
You are on the right rack in trying to find constructive ways to improve your game.
My suggestion is to try and locate a competent instructor in your area to have look
at your game. Even if you cannot find one in your immediate are and you have to
travel to see one, it can be worth its weight in gold.
My very best wishes to you!