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.
If a slinky is not at the top of your Christmas list you might want to think again. Making a slinky walk down steps or draping it over your arm to imitate a robot is fun but the coiled spring toy hides a more intriguing capability.