I can't comprehend how so many have an issue with this. It's such an intuitive thing. I've never had a single issue since day one.
You don't need to get anal about pointing your toe, lining up with the roundel or thrusting your leg in a certain manner. Just walk up and give a quick kick in/out. The sensor is up behind the bumper. It measures the longitudinal movement of your foot fore/aft. It does NOT use the park sensors.
Distance from the bumper isn't particularly important (vertically). It has enough range to reach the ground. It also has a pretty reasonable range side to side.
I literally just walk up and give a quick kick. Works 100% of the time. Don't overthink it.