This is most common if you step on a slippery surface. Your feet get swept forward leaving you suspended for a second in the air then you come crashing down on your bum or worse back.
For learn we start by reversing the fall, and using the spring method.
Lie down on the floor.
Tuck your chin into your chest to stop your head rolling back and hitting the floor.
Raise your back an inch and let it drop back to the floor.
Feel the potential damage that could be done if you raised it higher,
Now try it again, except this time,
Extend both arms out and slap the floor as hard as possible with the palms of your hands, as soon as your back touches the floor.
Notice the spring effect bounce. When you are comfortable with it, increase the height of the fall.