Suizen: what is it?

Suizen (吹禅) is a Zen practice consisting of playing the shakuhachi bamboo flute as a means of attaining self-realization. The monks from the Fuke sect of Zen who practiced suizen were called komusō (虚無僧; literally "emptiness monks"). It is believed some suizen melodies were used as identifications. This was to help identify Ninja. The melody would be played and if the second komusō did not respond, the first would know that the other was probably a spy