What I'll add as someone who is doing it in part of an Open Samba routine but not 100% proficient...it is one of the easier steps I've learned. Won't say I've mastered it.
I tend to agree that the swivel comes from the placement of the back foot, but there is also an almost imperceptible and nauseatingly quick double knee rise on every step. A tiny jerk forward and than the actual movement to place the foot back, of course no weight replacement until the full thrust backwards, and step is done entirely on the ball of the foot at least for the lady (movement is too fast for putting the foot down and up again).
I'm not particulalry technically proficient in dance...so I may not be describing well.