A Bujinkan formula: K(p+t)(s)=K
Let's ignore K (ku) for a minute because I'm not sure that can be taught.
I suggest that: PLAY = SHIZEN.
so p+t=h combined with s OR,
PLAY using TAIJUTSU to produce HENKA from a state of SHIZEN.
If that hurts your brain let's forget it and consider this:
When I was 13 years old, Sensei stated while teaching in America:
My techniques are adaptable to any situation. It is from improvising action as it flashes into my mind. My techniques are natural techniques.Improvisation creates natural techniques. And now that I am 40, Sensei is still saying play!
Baby animals when they play are learning how to fight and hunt. Their play is often very amusing to watch. It is so free and unencumbered by self consciousness. It is also improvised where anything can and does happen. This serves them well for a real fight or hunt. Adult animals keep this spirit alive and react unpredictably when cornered.
The opposite of play is memorization and rehearsal. This just traps us in our movement and in our own heads. It creates stifled, robotic action that is easily defeated by unpredictability. Like an unexpected math test!
Play is something that comes naturally. You don't need to force it or learn anything to accomplish the state of playing. In fact, it works best when you just let go. Or improvise.
Hatsumi Sensei still uses the word PLAY often. It is not a command so much as a cheer. It is meant to inspire and embolden. To re-ignite that spark in our hearts that is alive like a puppy or a child at play. Maybe that spark arises from K (ku)?
Understand? Good. SHIZEN!