Simplified:
The turning motion of your wrist applies rotational inertia to the axle and bulbs.
Inertia builds (stored as angular momentum) as the string unwinds.
When the yo-yo string is fully extended, friction acts between the string and axle to resist further spinning motion (and the downwards pull of gravity), and potentially ‘catch’ and re-roll the yo-yo… depending on the motion immediately following the formation of this disagreement in forces, one of two things will happen:
1. Angular momentum will keep the axle spinning no matter what… if the yo-yo is left unassisted at the lowest point, gravity will keep the axle down at the end of the string, “sliding hard” in circles, until it runs out of energy or:
2. A slight motion on behalf of the user can cause the yo-yo to mometarily stop “sliding hard” on the string and the spinning motion will simply grab the string and begin re-winding it on the axle… as soon as the string has begun rewinding, friction kicks in hard and heavy and stops relative sliding of string and axle, causing angular momentum to wrap the yo-yo back up its string, against the pull of gravity until it is again fullly wrapped.
In order for a yo-yo to go all the way down and up, enough energy must be stored in angular momentum to overcome the potential energy of lifting a yo-yo’s mass the distance of the length of string into the air.
