No matter how long the slinky is, the bottom of the slinky will stay still (hover) until the top reaches it. Even if the slinky is over 1000 feet long.
tension in the slinky between the top and the bottom form an equilibrium with gravity, meaning no change in position is caused as the forces are equal.
For anyone who wants to see the source video, it’s rad. Derek, in general, is rad.