All movement comes from one or more of the following 3 sources: Muscular power, gravity, and residual momentum. When we look at pure rotation on a plane parallel to the ground, we can rule out gravity, so that leaves us with muscles and momentum.

With quick spins from a static position, the power is generated at the beginning of the movement, and then the remainder of the turn is powered by the momentum from that initial burst of energy. Extremely small turns -- say, 1/2 or less -- are generally so small that you never get to the point where you relax and ride out the momentum. With these turns, your muscles are actively working to stop the turn almost immediately after they have initiated it. Conversely, with extended turns -- say, for example, doubles or triples (depending on the nature of the turn) -- you might need to regenerate some of that momentum with additional muscular activity to keep turning.

With the type of turn you describe -- a back swivel of approximately 1/2 turn on one foot -- it's really just a muscular activity, a release and a stop. So to your question, "Where does the momentum of the turn come from?", the very general answer in this case is simply muscular activity. In other words, there's very little, if any, force at work here other than the timing and positioning of your own body.

Now to be more specific about the action in your case, the back swivel is almost exclusively a twisting action, or really two twisting actions: One of the upper body and the other of the legs and feet. The secret is that they don't occur at exactly the same time. We teach beginners to hold everything together in one piece, because they need to learn how to contain everything before they can be taught to selectively disengage or offset parts of the body in ways that are beneficial. I say this because it's important to understand that ultimately it's not wrong (in fact, it's *more* correct) to turn "in pieces", so to speak, so long as it's done the right way. Also, it's important to understand for turns greater than 1/2 that the body would need to realign very quickly after the initial burst of speed, and this cannot be done without knowing how to properly align the body in the first place.

That being said, the offset timing of the twist of your upper and lower body ultimately makes this turn (and almost all basic turns, for that matter) much easier.

To begin, you can teach yourself where these two twists "come from" (i.e. what muscles produce them) by simply isolating them and practicing them outside of the context of this turn. Lower body twist is essentially a form of turnout / turn-in. The turnout is of the entire leg relative to the rest of the body (not of the foot alone), so it occurs at the hip. There are two ways to do this: One is to turn the legs under the body (basically one leg turning out while the other turns in, as in the Chubby Checker dance called "The Twist"), and the other is the turning of the whole body over the standing leg. The latter is what we use for most outside turns, such as the pencil turn. But as it happens, this particular turn uses more of a leg twist (i.e. the Chubby Checker type). So to teach yourself where this part of the twist comes from, try dancing the Chubby Checker Twist and pay attention to what the muscles are doing to produce it.

The second twist is that of the upper body against the lower body. To teach yourself the action, place two fingertips of each hand on your sternum with elbows pointing straight out to your sides. Take a wide stance, and without turning your legs or hips, twist your upper body as far as you can from left, then to right. Remember that you must keep your hips still, twisting only the spine above the hips. Keep twisting back and forth slowly, to the maximum your body will allow on each side. This is upper body twist.

To put this in the context of your back swivel: The first action is a step back onto your right foot, making sure the weigh