Hmmm...
I worked with a guy years ago who was taking simultaneous measurements of voltage and current and given more time I think he was going to re-discover Ohm's Law. He was very fascinated by the proportionality which he had discovered.
I think this researcher has chosen poor words to describe what I think is happening.
Looking at the video there does appear to be a transition in the behavior of the spring when the coils close completely. The transition between the region where the coils are completely closed, and the region where they are not, does seem to travel downward along the length of the spring.
When this transition finally reaches the bottom of the spring, the spring force accelerating the bottom of the spring toward the center-of-gravity of the spring disappears. The bottom of the spring doesn't begin dropping because it receives a signal, it drops because it lacks the force which has held it in place.
Aside from some torsional effects, it does appear that the region of the spring below the collapsed part, is stationary.
I'm still struggling to imagine how such a non-uniformly extended spring would behave without a gravitational field.
I still find it fascinating that the slinky's bottom, essentially, doesn't start moving downward until the end. Even with a compressing center of mass, the slinky is so perfectly designed the energy in the tension equals the force of gravity.
The true explanation can only be found in God's humor.
thanks