Some people like to go round and round in their hanging chairs. Some like to bounce. Some like to do both. This yolk is the bit that bounces. It looks like it's been nicked from an old fashioned bedstead but that would be doing the German boffins who came up with this little slice of genius a dis-service.
Try using a couple of them with a hammock...wey-hey!
It's a nice big spring, 21cm (over 8 inches) long and well up for a bit of a stretch.
You're going to like it.