Daniel Hegerich  Description for marble in wok:

In training to connect one's mind with moving their coccyx, imagine a heavy marble is being held by the coccyx much like the marble resting in a wok. Picture the wok representing your coccyx and the long handle represents the connection to your sacrum. Now imagine slowly rotating your coccyx 360 degrees while holding on to the heavy marble with the handle of the wok being free to rotate about the sacrum. With practice you will then begin to sense the marble rolling around within the coccyx just like the marble is able to roll up the sides of the wok. Remember training your coccyx is a vital principle that should not be taken lightly. So stir fry some Chi in your wok daily!