The monk in this koan asks a question that underlies our early approaches to practice - what does the state we're trying to attain look like? Usually, this involves an element of subtraction - what is it in my mind I have to get rid of in order to be enlightened. Wiping the slate clean is one version of this fantasy. Tossing a ball on running water dashes this fantasy and is not at all our image of the calm meditative mind. It's a picture of the mind in motion. A mind moving so fast that nothing can grasp it. This is also an image of how Zen approaches consciousness. The goal is not to create the calm or stillness of the undisturbed lake. Instead, there's something we settle into that stops trying to control or eliminate anything about what our mind is naturally doing.