Making the enso—the Zen circle that conveys everything, the whole world, complete, the ultimate Zen symbol of emptiness—is kind of a practice. Ensos are traditionally done in sumi ink, black on white, but Kazuaki Tanahashi is well known for his colorful ones, which he makes with one brushstroke.
The Enso symbol means circle in Japanese. It is a spiritual symbol that has a very deep meaning, especially Zen Buddhists represents all, and the true nature of existence, the circle is ourselves. His circle is not closed in itself, but on the contrary, is open to the infinity.