Acoustic resonance bells, each ball vibrates at a different balanced frequency.

The size and composition of the Balls is important and the surface should be pure metallic so as not to impede the sound. 

The ball is rang like a bell and not knocked together as this can damage them.

It takes a little time to get proficient at producing the right vibration but when you do it will greatly enhance you meditation and stillness.

This is the original purpose of the balls, during the Mao Zedong years the focus was placed more on the physical health benefits.

These balls are authentic plain iron of the correct size and matched resonance. 

Feel free to ask questions.