This exercise helps get the feeling for the correct timing. Keep your arm firm, because she needs to push off of you. Obviously, this doesn't work with ladies who don't know the step, since they won't know to push off. Beginners with one step under their belt can't be expected to follow it, and you shouldn't try. When I dance with beginners, I sort-of 'test' for the American spin reaction by leading an underarm turn and adding a bit of tuck.
Beginners just follow the underarm turn and fail to compress or tuck.
OK, we just stick to basic steps. Since I still have hand contact the lead doesn't falter. Then I know I can use Tuck style moves. You can't just push the lady with the left hand because this will tend to push her backwards, when you really want her to stay in place and spin. However, there is a way to refine this lead so that ladies who don't know the figure can still follow it.