Bamkei is one of those Zen masters who you don't really hear about. He had a subversive style, amongst the Zen people of his day, because he seemed to be totally anti-practice. For him, Buddha nature had nothing to do with zazen, koans or even any sort of "mindfulness": he just insisted that whatever situation you are in, or type of person you are, you are Buddha right there and then. His attitude seemed to be that practice of any kind could cloud your understanding of your own Buddha nature. He didn't seem to insist on zazen, but his students did. He had no problem with people sleeping during zazen. He railed against the asceticism that Zen practitioners can fall into. He also left no official disciples.
"Practice" is the buzzword in Buddhist circles today. The familar koan about rubbing the tile having nothing to do with being a Buddha is wheeled out as a nod to the sort of untrammeled Zen that Bankei was spreading, but really everyone ends up still playing the Buddhist game, sitting for all their worth on a regular basis, wearing robes, building centres and so on and so forth. It's not that any of this is bad, but quite simply that the trappings can get in the way. It's easy to confuse sangha building and the "Zen game" with a real enquiry into Self, life and death. This is what Bankei pointed out.
I would recommend both Peter Haskel's and Norman Waddell's books about this one-off Zen great. Hurrah for Bankei!
