If the pen settles behind the midpoint of the saddle, the balance is cantle low. More weight will be carried by the back of the saddle and the corresponding area of the back, instead of being evenly distributed through the entire panel of the saddle. On the other hand, if the pen settles forward of the saddle's midpoint, the balance is pommel low. This will create a pressure point under the front of the saddle. In this case a deeper gusset is not needed, but a wider tree.
Placement is also key in assesing balance.
When the saddle is placed too far forward on the shoulders, it will typically sit cantle low. This is easily corrected by sliding the saddle back behind the shoulder blade. This is evident when you alternately press on the cantle and the pommel. A saddle that rocks will frequently move forward as the horse works. In this case, the horse's back will curve more than the panel of the saddle.