The problem is a bit hard to tell with the naked eye just what’s happening.
He explains that gravity pulling the bottom end down, and tensions pulling the bottom end up.
The two forces are equal and opposite so the bottom end remains at rest.
Then he let go at the top end and the tension in the spring changes, but it propagates down the spring coil by coil until it reaches the bottom end.
And then the bottom end falls. So the tension doesn’t actually change at the bottom end until the rest of the slinky has collapsed.
a navel engineer was trying to make a meter designed to monitor power on naval battleships. He was working with tension springs when one of them fell down the stairs. The spring kept falling down the stair continuously (like a slinky does) and the slinky was born.