The performer exhibits a ball several inches in diameter. S/he pokes a knitting needle through it to prove that it's completely holed; s/he threads a piece of string through it and then allows it to slide up and down. But, on command, the ball will slow down and will even stop.
The diagram gives it away: the string is actually threaded through the curved channel, and of course will move or not depending upon how tight the performer keeps the string.
The string channel will have to be substantially wider than the string, and the latter will definitely need to have an end like a shoelace otherwise it will never thread! (The little shoelace-end gadget actually has a technical name but we can't remember it!)