For most people, the hardest part of yo-yoing is getting the spool to sleep long enough to pull off some tricks. To get an ordinary yo-yo to sleep for a while, you have to throw it with a lot of force so it builds up strong angular momentum. But when you throw a yo-yo fast, your hand tends to jerk, pulling the spool back in. Beginning yo-yoists also have trouble "waking" a yo-yo (pulling it out of a sleep). It takes a lot of practice to get the right balance to put the yo-yo to sleep successfully.
Yo-yo manufacturers have tried a number of things to make it easier to keep a yo-yo sleeping, and to make it wake up again. One of the simplest improvements was to redistribute the weight in the yo-yo in order to alter its moment of inertia.
An object's moment of inertia is a measure of how resistant it is to changes in rotation. This is determined by two factors: how much mass the object has and how far that mass is from the object's axis of rotation. Increasing mass makes an object harder to rotate and harder to stop rotating, as does increasing the distance between the mass and the axis of rotation (a rolled-out slab of clay, for example, is harder to spin than a tight clay ball with the same mass).
If you increase the moment of inertia in a yo-yo's discs, the yo-yo will be able to sleep longer; it takes more work to stop the rotation. For this reason, manufacturers often concentrate the weight in high-performance yo-yos around the outer edge of the spool. Since the distance is larger between the axis of rotation and much of the mass, the spool will have a greater moment of inertia.
Another approach is to further reduce friction between the yo-yo string and the axis. One popular method is to configure a ball bearing assembly around the yo-yo axle, so the axle itself is separated from the string. You can see how a typical bearing system works in the diagram below.
The bearing assembly consists of two races, essentially grooved tracks for ball bearings. The inner race immediately surrounds the axle, and the outer race is spaced a bearing's width apart. The ball bearings are positioned between the two races. The yo-yo string is looped around the outer race, so it never touches the axle itself. The races are not bound together: The inner race can tilt slightly inside the outer race.
When you throw the yo-yo, the unwinding action spins the outer race. The force of the throw tilts the inner race inside the outer race, which increases the friction between both races and the ball bearings. Effectively, the tilting action locks the races together, so they turn in unison. In this way, the spinning outer race spins the inner race, which spins the yo-yo axle.
When the yo-yo reaches the end of its string, the gyroscopic motion of the spinning discs tends to level the races out, so they are aligned with one another. With this configuration, the ball bearings can move smoothly between the two races. If the bearings are properly lubricated, they will significantly reduce friction between the two races and protect the bearings.
To wake the yo-yo, you jerk on the string. This tilts the outer race in relation to the inner race, increasing the friction on the bearings. The spinning motion of the outer race carries the yo-yo back up the string.
This mechanism makes it easier to keep a yo-yo sleeping, but it doesn't help much with waking the yo-yo up. In the next section, we'll look inside the new "automatic" yo-yos that sleep and return on their own.