The magician displays three ropes in his closed fist, offering a cash prize to the spectator that can select the rope with a secret marking on it. Despite his best efforts, the spectator's two selections are NEVER correct. The specetator pulls two ropes from the magicians fist, and neither has the secret mark.
Offering to show the spectator's what they were missing, the magician opens his hand. The secret mark he alluded to was, in fact, a dollar bill (or whichever denomination you can afford) tied to the rope! The magician removes the bill and unties the knot.
The trick is self-working, and comes complete with three hand-knitted ropes made in Europe. There is no equivoque or forcing - the spectators really do have a free choice of ropes.