Complete Sincerity
When a samurai has said he will perform an action, it is as good as done. Nothing will stop him from completing what he has said he will do. He does not have to "give his word." He does not have to "promise."
Speaking and doing are the same action.
This means both doing what you say you are going to do, and also realizing that your words are valuable and not saying that you will do something unless you really mean it.