Used literally for hundreds of years, these hand exercisers have been used by the Chinese to improve their Chi flow. They are called Baoding Balls or Chinese Exercise Balls. There is a long history inhert in these balls with Yin and Yang, your chi flow, and healing benefits. Still being used today, many tai chi practitioners use them during their exercises.
But for dexterity and coordination building, they are a great exercising tool. With two metal balls, you rotate them in the palm of your hand using your fingers to direct the motion and speed. It takes coordination and practice to be able to spin the balls without them hitting against each other. Once you build up your coordination, you can move on to more advanced exercises such as spinning them in the opposite direction or spinning three balls in the palm. To increase difficulty, you can advance to heavier and larger balls.