Here's a great article about it.
also, it's important to note that there are different mechanisms that drive these polishers. I use a PC7424, which is a passive dual-action polisher. It has a motor that oscillates the head, but the spinning action is a secondary effect of that primary motor.
Other polishers, like the Griot's (I believe) are true dual-action polishers in that they have an active link to the motor which guarantees you get the same action regardless of the amount of pressure you're putting on the pad.