The conventional Foster slug is stabilized in flight much like a badminton shuttlecock. If you section one of them, you'll find a heavy nose with a large hollow "skirt" at the rear. They do not need to spin to remain stable.
A sabot, on the other hand, has a regular rifle bullet (usually a .44) inside the sabot. Once the plastic sabot has been shed (almost immediately out of the muzzle), the bullet will need to be spinning in order to remain stable. As stated, you can shoot sabots out of a smoothbore, but the accuracy will be miserable.