Originally Posted by Stephen McTowelie

Yup basically you change the angle of the blade to almost negative pitch and they speed up as you drop and then at the last minute you "Flare" by putting in lots of collective (positive blade pitch/angle) and with any luck you wont hit the ground to hard

Or you can try this upside down auto-rotation seen it in real life its extremely impressive but also expensive if it goes wrong.
https://youtube.com/watch?v=Jxakw5c9Ywo