When a stretched spring is unrestrained, both ends of the spring are pulled towards the spring's center. So what you are seeing is a 2 forces (tension and gravity) acting on the two ends of the spring with two different results:
The top of the slinky accelerates downward very quickly due to the combined force of gravity and tension
The bottom of the slinky stays approximately still because the downward force of gravity is balanced out by the upward force of tension
Once the spring finishes compressing, then the compressed spring is pulled to the ground by a combination of momentum from the top of the spring and gravity. Notice that even with the momentum of the top half of the spring "pushing" it downward, the compressed spring still moves much slower that the top half alone.
http://www.reddit.com/r/reddit.com/comments/l3auo/and_this_is_how_a_slinky_fa...