Ro-buki is the practice of blowing nothing but ro, the lowest tone on the shakuhachi. While playing that shakuhachi can be a form of meditation, there is no practice that is more meditative than this. You can close your eyes, play, breathe, play, breathe, and so on, as long as you want.
Nothing is more indicative of how well you can shape and play a note than ro-buki. It’s the shakuhachi naked; with no ornaments, no changes in pitch, nor in volume, ro-buki is the vanilla ice cream of the shakuhachi.
I only started doing this recently, following my teacher’s instructions, but I already see the value of it. It makes you focus entirely on the embouchure, the steadiness of the breathing, and the bamboo-leaf shape of the note. It’s boring; it’s the same boredom of shikantaza (just sitting) zazen. You do nothing other than breathe.
Of course, it’s not as simple as just breathing and blowing. The goal – if there is a goal in what could be seen as a goalless practice – is to make each breath, each note sound as it should. Here are some tips for practicing ro-buki; here are some more tips; and Kaoru Kakizakai has some recordings of him playing ro-buki on four different shakuhachis (1.8, 2.4, 2.7, and 3.2) that you can play along with on his website.
It’s interesting to compare my feeble attempts at playing clean notes with the recording by Kaoru Kakizakai. Viewing his audio files in an audio editor, and recording myself and viewing my files, shows the shape of the waveform. His, here, flows smoothly, with even volume, tapering off with a slow decay.
Whereas mine is a mess, showing how uneven my breath is:
This shows how far I have to go.