It's actually a pretty simple concept.
Speed related activities (swinging, throwing, sprinting) are a function of the muscles ability to contract eccentrically then isometrically. As the muscle "locks up" isometrically, the tendon stretches creating potential energy (static-spring). The stretch reflex will also fire which increases rate of force development (stretch-shortening cycle), but that's kinda beside the point based on this topic.
Therefore, isometric strength of the trunk is extremely important for rotational athletes (baseball, golf, tennis, volleyball, quarterbacks, etc.). The more effectively the athlete can contract the appropriate musculature, the more energy that can be stored in the tendon (assuming appropriate tendon compliance/stiffness) and then it can be released thus increasing swing velocity.
The bridge exercises are simply early stage exercises of a long progression which ends in high velocity reflexive contractions.
Now that's assuming your issue isn't simply relative strength. If you already have effective static-spring (which you may based on your 300 yard drive) and the ball is heavier (softball), the issue may be increasing your ability to produce greater force. Then look to increase absolute trunk and posterior chain strength especially the relationship between your lats and the opposite glute. This is where a great portion of rotational force comes from (if you look at fiber direction this relationship looks like one long muscle from shoulder to hip).
Ever see some of those big Mo Fo's that crush the ball...higher maximum force.
Does that make sense (I'm kinda draggin' tonite)?
Bill
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