Levitating slinky - GIF

[desbromilow]

Can someone explain that phenomenon please. Why does it happen?

from my understanding the centre of mass (CoG) of the spring is falling at the speed defined by normal equations of falling bodies. The top part of the spring is being pulled towards that CoG, whereas the bottom part is being pulled upwards towards the same CoG. Because of the light weight, and spring constant of the slinky, it gives the impression that the bottom is stationary and the rest of the spring comes to it before it starts moving.

least that's my understanding of what you're seeing - and that is why the dropped ball is not at the same starting height.