I think the most important Bunkai rule is to know as many applications as possible and be able to do them.

If you train a kata and you don't know what you are doing, then you're wasting your time. The kata must become reality.

You must know that every little step/attack/block can be done in many different ways, in different directions.

One of the best karate teachers I met said once to me "You know a kata when you know at least 1000 applications to every step in the kata". Perhaps "1000" is too much but I understand his point. In bunkai one movement can be done in many ways.
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Greetings

John Steczko

http://karatesoul.com