Note from Martin:

A spectator chooses a card and returns it to the pack. The magician disperses all the cards into a hat, then lowers a piece of rope into the hat. The magician continuously holds the rope's end, so keeping it in permanent view of the spectator's, yet on lifting it, it is seen to have tied a knot around one card. The knot is undone and card removed to reveal the chosen card.

 

Easy to perform yet visual and practical magic, plus supplied with bonus routine.