Hacker News Reader

I've seen it in a FB math group with the text: Applications of knot theory :)

I watched it 5 times and can't figure it out. Explain to me this sorcery.

This is topologically equivalent to pulling your hand out of the "cuff", letting the blue cord fall free, and then putting your hand back in.

Perhaps you find the reversed gif helpful: https://imgur.com/a/hwzePnS