I don't think it's complicated either, but it's still (at first) unintuitive.
Interesting things tend to happen when you have forces that cancel/counteract each other (e.g., spinning a bucket of water over your head, dropping a magnet in a metal pipe)
It's more intuitive if you think of what a much tighter spring (i.e. higher spring constant) would do. First, you'd have to hold it open because gravity wouldn't be sufficient to stretch it out. If you released both ends simultaneously, it would fully contract in much less time than it would take to hit the ground. As a result, the bottom would move up fast before moving down as the fully contracted spring falls.
The interesting thing worth noting is that you don't have to carefully choose a spring such that the force of contraction is equal to the force of gravity to observe a stationary bottom end, as in the gif. You just have to let any spring hang freely. Prior to release, the upwards force from the spring on it's own bottom exactly balances the downwards force from gravity. For a few moments after release, until the point when the spring contracts enough that it no longer applies the same upwards force to it's own bottom end, the forces on the bottom of the spring will remain balanced and no acceleration will be observed. This is, of course, much easier for the human eye to observe in slow motion and with a relatively large spring with a low spring constant, such as a slinky!
Interesting things tend to happen when you have forces that cancel/counteract each other (e.g., spinning a bucket of water over your head, dropping a magnet in a metal pipe)