The ride is a centrafuge, but at the end of the arm, your pod can be rotated too. This way, they can create acceleration / decelleration forces in various directions. If the pod rotates so you are facing the pivot point (inward), you get forced in your seat. If you are facing outward, you tend to feel like you are coming out of your seat. For me, the only obvious forces not felt are collisions with the ground. The forces can make it difficult to know your correct orientation, just like what happened to JFK jr while flying. All pilots know you believe the instruments and not your gut feelings.
I for one had no problem with MS, but you'll never catch me on Tea Cups again. Even Dumbo will make me more dizzy than MS.
I'm surprised so many people do actually go on MS, considering how many warnings they give you. It is fun to watch first timers reactions.