The horse’s back doesn’t bend in a continuous curve.  Basically, it folds in the region between T9 and T16, which is right about where the rider sits. There is a very small amount of bending in the lumbar spine. There is also rotation in the thoracic spine which can be felt as being tipped to one side or the other when the back is not well coordinated. Bending and transverse rotation always occur together. Healthy transverse rotation is occurring when the dorsal processes tip very slightly toward the inside of the bend.  Inverted (unhealthy) transverse rotation occurs when the dorsal processes tip toward the outside of the bend. Over-bending the neck is one thing that causes inverted transverse rotation. Muscular imbalances can also result in inverted transverse rotation.