Everyone knows the expression “It’s like riding a bike”. Once you learn how to ride a bike, not getting on one for a year or two doesn’t mean you’ll have trouble the next time.
However, if you still haven’t mastered riding a bike, then don’t practice for a year, you‘ll have to start over from scratch. In other words, the phrase“It’s like riding a bike” only holds once you’ve already gotten the hang of it.
The same thing can be said of practicing songs on the shakuhachi. If you concentrate your practice during a certain period and make a breakthrough(often a successful performance in front of a lot of people will help with this) on a given song, you will no longer have to practice consistently to maintain your level at it.
Once you make this breakthrough, you can not play this song for years and then pick up practically where you left off. If you haven’t reached the point of breakthrough, though, leaving off practice for such a long period will force you to make up a lot of lost ground.
This is particularly true of honkyoku. For songs where there is a set rhythm and all you have to do to follow it, maintaining the “feel“ of the song is neither very necessary or vary hard. Honkyoku, however, have an elusive ma(“absolute timing”, or “the space between the notes”) that you need to have a good handle on for the song to be effective.
It’s like swerving suddenly on a bike to avoid something and then just barely regaining your balance. It’s not something you can think about – it must be instinctive. The same thing can be said of “ma” in honkyoku.