The narrator in this clip asks all people “How do you explain them?”. And them cann't explain. Well to help explain is physicist Rod Cross. He holds the top end of the slinky and then let the bottom end dangle. And he would to let go it. The problem is a bit hard to tell with the naked eye just what’s happening. He explain that gravity pulling the bottom end down, 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 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.