The following information comes from The Elusive Ten by William Reichenbach. The Elusive Ten is also available to purchase in print.
It happens sometimes that the group of your shots appears elongated horizontally. (If this distortion is caused by body sway (see First Chapter under “Stance”), then we have an effective remedy.)
By advancing your rear foot slightly you change the sway of your body. Instead of swaying from left to right with your gun-arm, you will now sway more up and down, thus causing the group to constrict horizontally.
If the group should appear elongated vertically, follow the opposite precept.
Remember, if you present more of your shoulder to the target, your gun arm will sway from left to right. If you present more of your chest to the target, the arm will sway up and down.