This is the most famous of all rope routines in magic. The magician begins by showing three ropes, one small, one medium, and one large. The ropes are gathered together, pulled out straight, and shown to have magically become the same size. The ropes are gathered together once again, and it is seen that they have returned to their state of different lengthedness.
Three ropes of different sizes are used. However, they are not really ropes but instead "ropes" made of wound rubber.
The ropes are legitimately shown to be of different lengths at the beginning of the trick. After the ropes are gathered together, the magician takes holds of the ends in each hand (all the left ends in the right hand and all the right ends in the left, so that the trick will appear correct from the audience's perspective) and pulls. The large rope is extended to its natural length, and the small and medium ropes' elasticity allows them to be stretched to match it. The only danger here is that if the magician loses grip on one of the stretched ropes, it may snap back with great force, perhaps causing a nasty bruise or (in rare cases) breaking a bone.
After the stretching, the ropes relax to their natural length and can be shown as different once again.
Trivia: The "professor" in the effect's name was Professor Waxman Mysterio (1886-1955), a traveling circus magician who never really mastered rope tricks even though he was contractually obligated to perform them. After struggling with this effect, he attempted to use one of the ropes to hang himself, but the rope broke and, due to his lack of rope-magic ability, he was unable to restore it.