In between the last rib and the hip is a small, but very important area of your horse. There are five major muscle attachments that converge here that are responsible for bending, the swing of the hind legs, lateral movement, and canter departs. If any of the muscles that attach here are tight, the hind end will not work correctly. When you see very tight movement in the hind legs, it may be that the area from rib to hip point needs massage. The affected muscles are the tensor fascia latae, the iliacus, the external oblique, the internal oblique, and the transverse abdominus.
If your horse is having trouble passing a jog, try having these muscles worked on. There should be immediate improvement.