"You don't just start tipping over your feet. Movement always starts from your center which allows you to compress through the knees, rotate, and push off the standing leg, with the forward or backward leg just following the movement of the body. Then again, everything moves so quickly, its hard to seperate things when watching, but it's true. Think about if you did a natural turn in waltz and the lady just stuck her left then right leg back. If she doesn't start the movement from her center, her body will still be in the way of the man. The legs move right after, but the center is always moving first."
Very well said.
I suppose the one exception to this would be the way that in preparing to step backwards when lowering from foot closure, the free foot moves back slightly from the shin as the feet lower. But the "real" movement of the free foot in that situation commences, just as in the other (non-tango) dances and actions, only after the body begins to move, and only as a result of the body movement.