Wow, that makes me want to try it, if only I had a high speed video camera....
Judging by reaction time of the guy looking down that entire thing is probably only about 1/20th of a second in real time, possibly shorter. Figure normal reaction time of about 1/100th and look at how long it takes for the guy to react to the other guy letting go.
I wonder if they added weight to the bottom of the slinky.
Edit: Reading up on this no need to add weight, it's all gravity and force vectors....