The inscription "Emptiness is form, form is emptiness" is most commonly used on portraits of Snowman Daruma. In this case, the inscription means, "Now you see me, now you don't." That is, "Out of emptiness my physical form appears, but my form will revert back to emptiness. That is reality."
Amabuki Sessan served as the general director of the Myoshin-ji school of Rinzai Zen. He was longtime abbot of Tokai-ji in Numazu. Amabuki wrote several books on Hakuin. Zenga by him are comparatively rare.