Chinese exercise balls are hollow spheres usually made out of steel, between 45mm and 55mm in diameter. The smooth surface of the balls is usually plated chromium or brass, often with some decorative pattern such as the Yin-Yang symbol. Inside each sphere is a number of small beads and a tuning plate that generates various tones when the beads hit it.
In aesthetic terms, even the plain chrome-plated style can be very attractive, and exercise balls are often kept in decorative boxes made of wood. Such boxes are sometimes covered in satin to add to the elegant look.
Generally, balls come in sets of two - one male, and one female, differing in tone. The most common way of using them is to place the two balls in one hand, and use the fingers and thumb to rotate the balls clockwise or counter-clockwise. This generates various tones as the balls move, and with practice it can be done in such a way that the balls rotate freely and smoothly without clacking together.
Exercise balls have been used for hundreds of years, often by Asian body-builders and spiritual seekers who use them as the focal point for meditation. It is said that using the balls on a regular basis can improve hand and arm strength, relieve hand pains (such as repetitive stress from typing), and even improve the functioning of the nervous system and memory. Some even believe that frequent practice with the balls can prolong your life, or at least promote better health in general.
With practice, one can work with three or more balls at once, although just two can be challenging enough even for those with rather large hands, depending on the size of the balls. It is also possible for skillful practitioners to rotate two or three balls at once without any of them touching, which requires rather fancy finger work indeed.
In a similar vein, lj pointed out the following: "Some people can handle five in one hand - four going round on the bottom, and one balanced on top, in a pyramid shape. With four in the other hand, and passing back and forth the effect is rather mesmerizing."