The performer shows a small cylinder with a hole running through it.
At the bottom of hole an elastic band is threaded across the cylinder .
A small plunger with a hook at one end and a cap at the other is pushed into the cylinder, and twisted by the performer, apparently threading the elastic band over the hook.
When the cap is pulled out and released, the plunger snaps back into the cylinder by the action of the elastic.
Watch the video demo: