[82Rigger]

Under a properly opened canopy (T-10 military chute) your rate of descent is about 17 feet per second.
Ground impact is about like what you would feel if you stepped off a 4-foot high wall.
The idea of a PLF is to transfer the DOWNWARD energy of impact into HORIZONTAL energy. Montana listed the five points of contact. In addition, you keep your neck muscles taut so your head doesn't snap backward into the ground. The situation is complicated by the fact that you are almost never coming straight down. You are drifting horizontally (due to even a slight wind) in one direction or another. You have to be able to do your PLF in the direction of your drift.
When I went through jump school and we were practicing PLFs the instructors had a thing about hearing that helmet hit the ground. When they heard that, they would stand you up and ask you if you knew that head hit the ground.
They would help you remember by fetching you a horizontal roundhouse to the side of the helmet.

Airborne! Steve / 82Rigger