The "jumping cloth" is a comedy trick with which you can make your audience furious.
You present a black bar decorated with a silver mica strip, through which three ropes are pulled vertically and immovably. On the right rope there is also a cloth knotted. You now claim to be able to conjure the cloth from the right to the left rope.
You go behind your back with the bar and when you bring the bar back to the surface, the cloth is actually on the left rope. However, the audience is not really convinced of your magical abilities, as everyone has seen that you have simply turned the bar behind your back. Some very clever people of course believe that they can trick you, the magician, by demanding that you let the cloth jump onto the middle rope.
What do you do now?
You hold the bar loosely in your hand and blow once on the three ropes. At the same moment the cloth jumps onto the middle rope, clearly visible to all, and without any cover or frippery.