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.
Consider… If an even heavier weight than the tennis ball was attached at the bottom & the spring was allowed to stretch to the greater length required to support the weight. And then the top was released ~ the weight would remain motionless until the upward pull lessened. The upward pull would remain sufficient to support the weight until the collapsed spring hits the weight.