So I conclude that the Beachskater's Single Push is driven by upper body muscles and not by gravity or the leg muscles. In this section I will argue that the two types of single push shown above can be combined synergistically to form a double push. But first let's see Chad do a double push. The blue line through his upper body looks for all the world like a gravity-driven pendulum as Chad's double push motion is so smooth. But below I will sketch what I think really happens here.
Gravity pushes down and the leg has a component which pushes to the side.
The ground reaction forces are shown for gravity and for the leg push. As I look at the video clips of various skaters I find all three methods of driving the second push in use. The skater's upper body model is shown on the right. Here T came from an analysis of Chad's video ground tracks. The full cycle time of his stroke was 1.26 seconds if the video camera had a standard frame rate of 30 frames per second.