Video: Even the most basic yo-yo tricks depend on the toy's ability to keep on spinning. But why does it spin so fast for so long? The magic centre of a yo-yo is a bearing — a bunch of tiny metal balls trapped between two rings. Because there's less surface contact between the balls and rings, less friction is generated than if were just a string spinning around a shaft. It's a simple device that's easy to overlook, but its crucial to a yo-yo's operation.
Part of what keeps the yo-yo spinning for so long is also the weight of the frame. Heavier materials act like miniature flywheels, adding inertia, but they only work if both sides are the exact same weight. That level of accuracy is why pro-quality yo-yos can often cost hundreds of dollars.