From a pitching rubber or any mark on the ground, batter goes through his pitching motion and marks where the toe of his stride foot plants. From that point, pitcher takes 5 heel-to-toe steps forward. Partner holds a target (usually a glove) above the position of the 5th step at a height equal to the batter’s eye level at his release.
Pitcher holds a hand towel between 1st and 2nd fingers such that about 12" of towel extends out. From the pitching rubber or mark on the ground, pitcher goes through his pitching motion and tries to hit the target glove with the towel.
This drill is good for practicing maintaining good posture and balance, getting the hips going, having good glove side management, getting good separation of hips and shoulders, and delaying shoulder rotation. It will also provide good feedback as any miscues will cause the towel to miss its target. Note that if a pitcher just can’t hit the target at the stride-plus-five distance, then they should shorten the distance - it is better to perform this drill as perfectly as possible than it is to hit the target.