Weight rings are circular weights which made out of rubber, plastic or even metal. They allow the yo-yo to have a greater proportion of its weight on the rim, resulting in longer spin times. Weight rings may also be inserted on the inside of a yo-yo to make it loop more efficiently.
In physics, anything with more mass has more inertia (the ability to resist a stopping force). When it comes into play with objects moving in a circular motion, more mass on the outside of that object makes it harder to slow down that object. The more weight on the outside rim of a yo-yo, the longer it will spin with less force.
Although rim weights may increase spin time, the sudden changes in speed and direction of travel of looping play can make rim weights unstable, making for a loss of speed and making the yo-yo spin maybe half way. This is where inner weights work better.
For weight on the inside of an object, the object's speed can be changed easier. In looping, there never really a is constant speed at which a yo-yo spins. the more weight on the inside makes changes in speed and direction of travel easier, creating a more stable yo-yo.
Although there is more mass and an increase in spin time, other things can slow a yo-yo down such as string friction and lube in the bearing.
The most well known rim-weight rings are those used by YoYoJam. Their weight rings are generally made of aluminium or titanium, attached to a plastic yo-yo hub in such as way that they do not come off the yo-yo. This method of attaching the rings to the yo-yo has been patented by the company.
Weight rings can be added to some yo-yos by inserting them into the yo-yo halves. Examples of this include the Yomega Hyperwarp Wing.