Cafagna appears to drop down on his left foot for several frames. In reality, the camera operator zoomed in a bit after the first few frames giving the illusion that he is rising up for the rest of the frames. The drop occurs when the images loop from last frame to first frame.
 
In all of the frames, another racewalker is visible on the right side. During the filming of the source video, Cafagna was training with Giorgio Rubino of Italy (visible) and Jefferson Perez of Ecuador (not visible in these cropped images). Cafagna was selected for this study because only his image is fully visible for a full gait cycle.