The other day after class someone asked me, "Where did you learn that technique?" To which I answered, "I stole it from Sensei." With what was probably a huge grin, I elaborated, "He showed it one time and I stole it."
As a Westerner, I think he was surprised that I would admit to "stealing" something, but in the East this idea of stealing is called nusumi-geiko and thought of as the highest form of learning and that's why I proudly admitted to stealing it from Furuya Sensei.
In the past, the teachings were supposed to be closely guarded secrets that were passed down from generation to generation. Outsiders were always met with a certain amount of suspicion and even more so if they had any amount of talent. Teachers were always afraid that someone might come in and learn their secrets and abscond off with them which might lead to attack or the closing of the school.
To safeguard these teachings, teachers often left crucial and often subtle things out that only a truly dedicated student could figure out with time and diligence. Sensei even once told us a story about a Chinese martial arts teacher who purposefully taught the form backwards to safeguard it. Only after he fully trusted the student would he then reverse the direction.
Teachers of old believed that anyone could learn their arts up to a certain level, but only a truly gifted student could master them. To determine the wheat from the chaff the teacher needed a method to determine the inner character of the student. Hiding the technique was one of those ways. If a student could pick it up on their own i.e. by stealing it, then they were probably someone of merit. It forced the student to not only be diligent but to be resourceful enough to "purposefully" steal the technique from the teacher.
An average student can learn anything, but only a great student can learn everything.