Banpen fugyo means not to be surprised when something quickly changes, or a new development happens.
An example might be you are working with a single training partner and suddenly two more appear.
Or in another moment your partner suddenly draws a training tool. Surprise happens when you focus on the wrong thing- your mental point of view is all in on that one moment, so when something else appears and you are forced to adjust it manifests as a surprise.
Surprise is dangerous, because in that moment the flow of your mind has stopped, and as a result your body has stopped moving- the very thing the chi ryaku no maki is trying to teach you, and at this point you should know and be comfortable with.
Being hyper focused is also bad because you will miss opportunities, both on a technique level, but also on a strategic level. For example, if you are busy cranking on muso dori to your training partner, and are so focused on making the lock “work”, you might miss a shift in their spine and footwork, that if you then switch to hon gyaku in that moment would be even worse for your training partner.
So how do we cultivate banpen fugo in our training?
On a physical level your techniques should only be done at 80%, never go all in or commit 100% as then you will be locked in and can’t change to anything that develops- catching you by surprise.
By having a 20% reserve you can easily adapt to any changes.
NOTE: This and the next statement does not equate laziness or carelessness on the part of the student.
“Thinking” in the martial arts is bad, and at this point in your training the thinking should be put aside so you can really experience what is going on. At the kyu level you are learning how to move your body so it is natural to break down a technique by steps- fist I step here, then I do this with my arm, then move this way, etc.
At the shodan level this is counterproductive- if you have to “think” about what you are doing your mind is stuck there, and when something goes off script, or a mistake is made, or your training partner moves in such a way that is new or unexpected your mind will focus and think about it causing your body to stop moving. Just keep moving, and always finish a technique even as mistakes are made- never “start over”, or second attempt it. From start to finish just move.
By letting the techniques and transmission of movement of the Bujinkan power you, your mind will be freed and not surprised leading to banpen fugyo.
As a side note, the book Unfettered Mind by Takuan Soho offers another easy to understand perspective of this cultivation we are all working towards.