Shaft flex can have a significant effect with your ball striking, we all know this, but shaft flex can also have an effect on what type of swing you have.
There’s more to consider than, what is the right shaft flex for your swing profile? You should also take into consideration, what is the right shaft flex for your acceleration profile.
If you have an early release or cast the club, even if you have a faster swing speed, you can most likely get away with a softer flex shaft. If you create and hold significant lag, even with a slower swing speed, you can most likely fit yourself with a stiffer flex shaft.
Taking these points into consideration your shaft flex will be critical in helping maximize your consistency.