So which is it? The result may surprise you.
First, make your bets on what you think will happen.
A Slinky is a loose tension spring. If you let the whole thing uncoil and hang down, the tension is enough to hold up the bottom against the pull of gravity.
Because the Slinky is dropped from the top, Cross explained in his study, it takes time for the motion wave to travel down the spiral and "communicate" to the bottom part that the top part fell — and that the tension is no longer there.
What's interesting to note about this phenomenon is it's not just a property of Slinkies. It's a property of all objects. You can have a really long steel rod and when you let go of the top, the top really starts accelerating down first, and the bottom second. It takes time for that relaxation to travel through any material. You need that compression wave to basically pass through the entire object. A slinky just makes that nice and visible for us.