I've played around with it, I was able to recreate it. It's the speed of the rope. If you let go slowly, like you would with one hand, the runner stays disengaged but your hand is on it. you quickly reach full speed and the Rope is moving 3 times faster than your falling, you can lose control, especially bare handed. Try it low and slow. You can try to grab it but it's like trying to stop a moving bag of cement by a rope but the rope is moving three times faster than gravity. Because you've panicked, you are maintaining enough pressure on the runner to keep it disengaged but not stop yourself. If you let go and take off all pressure from the tail, you will stop. I have also recreated this.