The loose loop lets the yo-yo keep spinning
A yo-yo’s string is attached to your finger, but at the yo-yo end it forms a loop around the axle—the rod at the center of the two halves. Because that loop is not knotted tightly to the axle, it can slip around it instead of forcing the whole string to move with every turn.
As the yo-yo unwinds, the string’s pull makes the yo-yo rotate. When the string runs out, the yo-yo can remain at the bottom spinning while the loop slides around the axle. That pause is called a sleep: the yo-yo is still turning, but the string is no longer winding it back up.
A tug turns spinning into a climb
The yo-yo does not rise simply because it is spinning. A pull on the string tightens the loop and presses it against the axle. Friction then gives the string a grip on the rotating yo-yo, winding the string back around the axle and drawing the yo-yo upward.
The timing matters. A gentle tug can wake the yo-yo from its sleep; a tug made while it still has enough rotation lets it wind up and return. A hard, straight throw helps it reach the end of the string with energy to spare, while the loose loop gives the player a moment to choose when that energy becomes an ascent.
