Yep, it\'s a cool trick.
It\'s a currently sold commercial item, but I agree that it\'s easlily made at home if you own a reel. A so called \"Stop-reel\" would help, but one can go even without one with few immagination...
Won\'t go any further: it would became exposure!

a friend of mine showed me a good homemade reel he uses with invisible line made with two straws, toothpicks, and a rubber band. pm me if you want details.