Kujira ochite iyo-iyo takaki o age kana

The diving whale—
how its tail keeps going
up!

Iyo-iyo here is a rendering of the Chinese po-po.
Huineng, the sixth Patriarch of Chinese zen used it in the 13th line of his Formless Gatha. Yung Hsi, one of his translators explaines it as “running hither and thither. It’s like waves that intermittently push those forward that are in front of them.”

Both Iyo-iyo and po-po have the Hybrid Sanskrit word ayavyaya as a source. Ayavyaya means coming-going.
The Lankavatara Sutra, one of 3 seminal zen-texts has in its 50th Chapter “I do not teach materialism, nor coming-and-going (ayavyaya)”. With these words the author critisizes the materialist Lokayata-trend of Indian philosophy.
The Sutra on the Unlimited Life of the Threefold Body (transl. Paul Swanson) also speaks of ayavyaya when it says that the Original Buddha neither comes nor goes.

Buson doesn’t come accross as a zen priest, but as a pupil of haiku masters who knew their zen sources and stock phrases he must have been aware of at least Huineng’s use of po po.