I know what this is, but how do you use it effectively if one is attempting to match the plane of the swing to the plane of the pitch? Thanks.
Here's what I do. When I set up the line, I will usually anchor the end by the hitter around the knees of thighs. I hold the end you pull around my stomach, which sets up a downward plane.
From there I have the hitter take a swing without using the ball. Her goal is stay connected, then have the bat lightly slide along the string for as long as she can. Once she reaches extension, she finishes. Sliding the bat along the swing demonstrates getting on-plane.
The next step is to shoot the ball at them. Amazing how many young hitters can get on-plane without the ball but swing under the string when the ball is coming. Most likely it has to do with dropping the hands to swing across instead of staying connected to the back shoulder and swinging from the top down.
Hope that description helps you visualize it.