Reflecting on the post below, when I was at Fort Riley Kansas, they taught us to rappel. First down a relatively steep cliff, then down a big board wall, then from the skid of a Huey.
The Huey was just barely hanging in the air with a full load of eight infantrymen. You could feel the airframe move whenever someone shifted their seats. If you rappelled from the aircraft, you'd rappel in pairs, one from each open door of the aircraft. Your rope was 50 feet long. So, when it was your turn, you'd get on the rope, stand on the skid and look at your buddy across the aircraft. One of you would nod, and you'd both step off the skid, fall about 10 feet, then stop your descent with your brake hand, swing under the aircraft, then on the outward swing, release your brake hand and continue toward the ground. Nothing to it, easy-peasy.
The only problem was that as the load lightened with infantrymen going to the ground, the aircraft would lift slightly as the load went away, two grunts at a time. If the pilot wasn't paying attention, the aircraft would lift slightly, and eventually, the end of your 50 foot rope would leave the ground.
I don't care how fit you might be, how quick your reflexes, how lightning-fast you can react to a situation. When you feel the end of the rope go through your brake hand, you are not fast enough to grab the end of the rope before you find yourself in free-fall. Trust me on this one. You ain't quick enough.
I never saw anyone get hurt, but I've seen lots of guys grabbing at the end of that rope, wildly trying to grasp it, before they fell that last foot, or five feet. For a bunch of twenty year old kids, that's high comedy.