With your favorite YoYo, deliberately throw a faulty "Sleeper"; that is, throw it so the YoYo hangs on the string with a definite tilt to the right or to the left. The inner face of one side of the YoYo (at the rim) will be rubbing the vertical string, slowing the YoYo spin. But watch the YoYo as it rotates its position to the left if it is rubbing the left rim, and to the right if it is rubbing the right rim.
The rotation to the left or right is called "precession" in the terminology of Physics. The vertical string, in contact with the spinning YoYo at the top of the rim applies a force that tries to bring the YoYo to the vertical position. Spinning momentum attempts to keep the YoYo spinning against the friction (with the string) at the rim. The combination of these forces causes the YoYo to "precess" - rotate to the left when tilted to the right, and rotate to the right when tilted to the left. Gyroscopes resist attempts to change the angular position of the axis of the spin.
When you throw an accurate "Sleeper", the YoYo hangs vertically with no contact of the string with either rim, and therefore with no precession from that source. If your YoYo is not well balanced, you may see some slow precession. Wood YoYos often have one side heavier than the other. If you have one like that, you might be able to guess which side is the heavier by the direction of precession you observe. The twist in the vertical string in combination with the weight of the YoYo can impart a slight torque on the axle and cause a slight and slow precession.
Here are some guidelines to improving your performance by avoiding or controlling precession:
1. Throw snappier "Sleepers"; the faster the YoYo spins, the slower it will precess.
2. In tricks like "Around The World" and string tricks, keep the string (or strings) in
the groove parallel to the groove.
3. In wood YoYos use fine sandpaper to remove enamel at the rim and to smooth the wood at the rim, then polish the wood at the rim with a hard wax, leaving a surface shine and no excess wax. This will mimimize friction between the spinning sides and the string.
4. Use a YoYo with much of its weight concentrated at the rim. The increased spinning momentum of these YoYos increases the duration of spin and also reduces the rate of precession.
5. It's always a good idea to keep your strings relaxed; that is, condition it from time to time to avoid excessive twist in the strands.