The problem: Horse throws hind quarters to the outside as he spins, thereby "swapping ends" instead of turning on the inside pivot foot.
Why does this problem happen? If the horse spins “like a top” (i.e. over his middle instead of his hind quarters), he’s not balanced over his pivot foot and has not been schooled enough in the basic turn around to want to do that. Probably he just wants to get around any way he can. He may be trying to hurry (or is encouraged to hurry) before he has mastered the basic maneuver.
How to correct this problem: If my horse is trying to spin like a top, I go back to basics. One thing for sure – he will not get better by practicing the spin incorrectly. I have to slow it all down, paying particular attention to the outside rein, keeping his body straight and encouraging him to lock his inside hind foot to turn. I may have to support him with a little more contact with the outside rein while directing him with the inside until he learns to pivot on the inside hind. (Beginner riders may not be able to tell if their horse is on the pivot foot so it is helpful to have someone watch.)
Exercise: I drive my horse forward in to a small circle keeping him straight between the reins (he may not want to be straight!) and bumping him with my legs for forward motion (he may not want to go forward either!) until he is accepting of what I am asking. Then I change my body (sit down and open inside leg) to ask for a turn around, but continue to bump with the outside leg until he initiates a turn. When he goes into the turn I quit bumping but keep body aids on for the turn for one turn or so. Any time he loses position (does not turn on pivot foot), I go back in to the circle again. I would not ask for many spins at the beginning until his position is consistent and correct in the turn around.
Note: When a horse spins like a top, he will not score well. Also, it can be hard to ride, especially if he is spinning fast. With the center of balance in the center of the horse instead of over the pivot foot, centrifugal force can throw a rider out of balance – even off the horse!