Here are the "sensor wires" that are mounted behind the bumper.
Each sensor will detect the movement of your foot (or any object) as it passes under it them. The sensors can detect the direction of movement of your foot by evalutating the timing of the trigger events from each sensor. The motion itself is not what triggers the opening, it's the rapid change in direction. You need to make a pretty quick and purposeful in/out motion, not a lazy wave. It's not like a typical motion sensor. Just moving your foot around isn't going to trigger it. Thrust with purpose!