This video features the “swing plane” of a high level hitter. It shows that the bat is on an up-slope in the contact zone. This is in contrast to the “swing down” swing plane taught by many baseball and softball coaches. The bat’s trajectory does start on a “downward” path, but as the video points out, it bottoms out about 18 inches from contact and is on an up-slope to the ball.
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Linear vs Rotational Swing Plane
Both linear and rotational hitters start with the meat of the bat higher than the back-shoulder. And since contact can be made below the belt, it is obvious the bat must be swung on a downward plane before it starts in an upward path. The main point of disagreement is where in the swing plane the bat bottoms out its trajectory. Many linear hitting coaches believe the bat should still be on its downward plane at contact.
Rotational hitting coaches instruct batters to keep their hands back and accelerate the bat-head on its downward path back behind the back-shoulder (back toward the catcher). The path of the bat plane will then bottom out about 18 inches from contact and be on a slight up swing in the contact zone. This video shows and discusses this swing plane.
Swing down to the ball may be a helpful batting tip for slap hitting where the objective is to use the batter's speed to beat out ground balls (i.e., for speedy left-handed batters to get a running start at contact). While these hitters perform a valuable service of getting on base, for most fastpitch hitters who want to drive the ball, the swing plane used by the hitters in the above video would be more productive.
Rotational Swing Plane Mechanics
Think of the plane of the swing as being a flat disc that is tilted down toward the plate so as to intersect the path of the ball in the contact zone. The bat, lead-arm and shoulders should all be in that plane from initiation to contact. The bat can be thought of as an extension of the lead arm. Keeping the lead-arm (including the elbow) in the plane of the swing is an absolute must.
That means the lead-elbow must always remain pointing into the plane of the swing. If the lead-elbow lowers (or drops) down out of the plane before contact -- the swing is basically ruined. The wrist will start to roll too soon and the bat-head will come out of the intended plane. This will normally cause inconsistent contact and usually results in weak grounders or pop-ups. Below is a video that shows a frontal view of the swing plane.