This reminds me of basic training when I was in the military. We started with the basics and went backwards over a 30' rock wall then I think it was a 90-100' rock wall backwards. When you do it the first time, the most unnatural thing was going from standing on the edge and letting the rope through your hands until you were still standing at the edge but now with your body horizontal to the ground and over the edge. We had a safety person at the bottom that would apply tension to the pair of ropes and would prevent us from sliding down if we slipped. I walked down the first wall. I took the second wall in 3 bounces. I got crapped on for being to cautious on wall 1 and then crapped on for taking wall 2 in too large of a bounce.
A few years later we were training again on a 50' rappel tower. It had a solid wall on one side to go down, and a huge pipe with no wall to simulate going off a helicopter skid. The first time I did it I went over backwards (back to the ground) and as per the instructors direction "remember to drop far enough or when you swing back in you will lose your teeth when you kiss the rail!" I think he said to make sure you drop about 6-10 feet to make sure you clear the skid. I did that and just clamped my hand to the rope and let it slam into the small of my back and I waited for me to stop swinging and then I lowered my self to the ground letting the rope slide through my hand.
The second time off the helicopter skid was a little more scary. Did it face first. This was with the concept of allowing a free hand to hold a weapon. I dropped straight to the ground. By the time I clamped on to the rope with my hand to stop me, I was too close to the ground for the safety person to apply any tension to the rope. I had too much speed, and my hand hitting me around my abdomen caused me to flip, which resulted in me standing on the ground. I colleagues were really impressed and said that was amazing. They did not realize how lucky I actually was.
Key to remember...Make sure you drop enough that you dont lose your teeth!
Also look at DudeOnRocks answer about having enough rope to reach the ground.