Among the different forms of Buddhism that got into Japan was zen (禅) and among the many groups of monks were the Komusō (虚無僧, monks of nothingness and emptiness), belonging to the Zen Buddhist sect of the Fuke-shū (普化宗).
To the sazen (座禅), the well-known sitting meditation common to most lineages of Zen, the Komusō added to their path of spiritual development an individual blown meditation practice named suizen (吹禅, blown Zen). This was created as a way of experiencing their motto ichi on jobutsu (一音成佛, Buddhahood in one sound).
At that time and in that context, the shakuhachi was neither considered a musical instrument, nor was the sound produced with it considered music.
The distinction between the aesthetical-musical ambit and the spiritual one was clear and strong as stated by the difference in naming the shakuhachi as hōki (法器, religious Buddhisttool) instead of gakki (楽器, musical instrument)
Monks were forbidden to play music with the instrument and its use was restricted to the function of spiritual development potentially leading to satori (悟り, illumination).
There was no aesthetic experience involved in the act of the articulation of the sounds involved in suizen.
The ‘sound organisations’, in lack of abetter name for these non-musical sound articulations, were not improvised. Instead they were transmitted to the novices by specific monks who, by the time of the banning of the sect were already, illegally, teaching the shakuhachi also to layman.
When the ban on the Fuke-shū became effective, the monks lost the exclusivity over the shakuhachi and a selection of these ‘sonic organisations’ was gathered and commenced to be transmitted freely and with no meditative purpose under the name of honkyoku, giving birth to the Kinko ryū ( 琴古流, Kinkolineage) in reference to Kurosawa Kinko (黒沢琴古)
Since the shakuhachi came into being in Japan as part of the import of court music gagaku (雅楽, elegant music) from China, this liberation of the spiritual use meant a coming back to the realm of being use with an aesthetic purpose.
Almost two hundred years have passed since the times of the blowing zen and conscious of the changes both in the pieces themselves as well as in their use, many honkyoku performers still relate to sound before music, choosing to keep in mind the original use of the instrument in the hands of the monks as well as the sonic characteristics resulting from that origin.
The ban of the Fuke-shū also meant the lift in the restrictions on the use of the instrument, allowing the shakuhachi to be included in other pre-existing musical repertoires as well as newly developed ones, first in Japan and later abroad.
Today a wide range of lineages and performers coexist, having different understanding of the instrument and the music made with it. Among others, these genres and practices include honkyoku, 民謡, minyo, 三曲, sankyoku, meditative forms, fusions of different kinds, jazz or Western Classical Contemporary Music.
For more information on these topics, see:
Sanford, James H., Shakuhachi Zen. The Fukeshu and Komuso, Monumenta Nipponica, Vol. 32, No. 4. pp.411-440, Winter, 1977.
Weisgarber, Eliott, The Honkyoku of the Kinko-ryu: some principles of its organization, Ethnomusicology, Vol 12, number 3, pp.313-344, 1968.