Eventually they teach you to do a fall from a full body height above the ground (~6 feet) which is ~22 km/h fall. Granted, the angles are diffent and forces are somewhat less. But, 20MPH with good gear and technique is generally not that dangerous.

Closest I have personally experienced was a fall off the back of a large horse onto hard packed dirt / loose gravel at a fast trot ~8 - 10mph + 5 vertical feet. But, with the right technique it was basically painless and I did not end up with a bruise or anything.