The "throwaway" is the movement of the woman's left leg, first under her body, and then well back to the point where the sole of her left foot is displayed to the world (we don't often show off this part of our costume).
Of course, the throwaway oversway is an oversway. We step side on the lead and relax that knee, while stretching the man's left side and rotating left to a closed position.
The man leads a throwaway by delaying the lowering action. Rotate LF first, and she will move her trail foot back. Then, begin to lower before she has a chance to change weight to that trail foot, and she can only keep moving it back, as you lower further.
If you lower as you rotate, she cannot move her left leg, and you get a plain oversway.