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Physical mechanism

When the yo-yo is first released, the gravity (and the throw) give it translational kinetic energy and necessarily, since the string must unwind, much of this energy is converted into rotational kinetic energy establishing the free movement of the yo-yo, and causing it to spin rapidly. As the yo-yo unwinds downward, it also converts potential energy from gravity to translational energy in its rotation to overcome gravity all the way back up to the hand.

Because the sense of spinning does not change during the whole move, the string winds up in the opposite direction upon the return of the yo-yo. If the shaft of the yo-yo is connected to the string with a loop, there may not be enough frictional force to overcome the weight of the yo-yo, which is necessary to begin winding up the string. In this case, the yo-yo will continue to spin in the loop at the end of the string (or sleep), just being slightly braked by the small dynamic friction, instead of returning. However, if the string is jerked slightly up, or the hand is lowered, the slack created in the string will allow it to begin winding around the shaft, thereby increasing friction and allowing it to catch as the static friction force rises above the gravitation force holding the yo-yo at the bottom of the string, making the yo-yo wind up the string returning to the hand.

Patents have been issued to create more complicated mechanisms to allow tension control and an adjustable mechanism.