This particular drill can be used for two completely different purposes, and trust me, it is very effective for both.
Hitting golf balls from your knees is very effective at reducing lower-body motion, and promoting a fuller upper body rotation – both of which “pushers” do poorly. By eliminating your lower-body movement, you’ll learn to speed up and rotate your upper body better. This move will coupled with less lower-body movement will help get your club on a better path into impact.
This drill is also very effective at fixing your slice. Swinging from your knees will force you to swing on a flatter path. The opposite, a steep swing path, is a common reason behind a slightly open clubface at impact. By swinging from your knees, you force yourself to flatten your swing path in order to make contact with the ball. Unfortunately, at this driving range, where this picture was shot, the range dividers were quite high. I would ideally like to make my swing even flatter than this picture portrays.