http://bendinggravity.com/new/wp-content/uploads/2014/10/Ignite-Spinning-1.jpg Rotational Energy More importantly, a Yo-Yo's Kinetic energy is contributed to rotational energy rather than translational energy. This is comprised from the Yo-Yo's moment of inertia and angular velocity. Since a Yo-Yo is basically a uniform disk, it has a lower moment of inertia than if it were a designed as a ring. This allows the Yo-Yo a to accelerate faster, but it does not allow for it to stay in the "sleep" state as long. Once the Yo-Yo builds up enough angular momentum, L=I$ω$ , it will have the energy required to spin its way back up the string to its starting point.$$$ω$ KE = ½ I $ω$² KE is the rotational kinetic energy   I is the Yo-Yo's moment inertia $$$ω$ is its angular velocity I = ½ m $$r$$² (solid disk) I is the Yo-Yo's moment of inertia   m is its mass $$$r$ is its radius ω = v / r ω is the rotational kinetic energy   v is the Yo-Yo's spinning velocity $$$r$ is its radius