https://www.wired.com/story/whoa-crazy-levitating-backpack/

This looks crazy. I mean, it’s a cool invention—the idea of the HoverGlide pack is to reduce the awkward bouncing and jolting you get with ordinary backpacks. But watching it makes my brain freak out a little. It kind of messes with your sense of how things should move.

The trick, I think, is that the bag is suspended on some kind of elastic cord. When your body moves up, it increases the force pulling up on the pack. But since the force increases only gradually with the elastic suspension (compared to the quick jerk you’d get from a stiff harness), it takes time for the upward velocity of the load to increase. By the time it starts moving up, your body is already coming back down.