Have you heard of "kukushi te?" This is the label for the "hidden hand" movements in the katas. These are the secret additional kata moves that only the high dans know.
We all show a little restraint when teaching killing or maiming moves. If the students are too young, or too sensitive, we skip over the dangerous knowledge temporarily and return to it another night when the audience is more appropriate. We don't teach knife fighting or eye gouges to teenage boys. They are too likely to use the knowledge inappropriately, and then who gets sued?
That's not "secret knowledge." That's just prudence. If you lean a little closer and listen carefully, I'll tell you the secret of kukushi te. Ready? There are no secret moves. Kukushi te is a myth that helps ignorant sensei save face.
Hard-style sensei are painfully ignorant of kata applications. Their best efforts are impractical and unrealistic. They know it. They lose face whenever they demonstrate kata applications. Do you think they would tolerate that humiliation if they knew better applications? They would not. They show us awkward, unreal applications because they know no others.
When put on the spot (by an inquisitive and therefore "disrespectful" student), they will improvise an answer. The easy way to invent a kata application is to insert "secret" moves into the kata. Anyone can explain moves that he made up himself out of thin air. There is no achievement there, and no honor. Kukushi te is a lie, used to intimidate students. It implies that they too will be initiated into these mysteries if they just keep paying dues long enough. There is no gold at the end of that rainbow, except for the sensei.
Every kata move can be explained in terms of jujutsu, or weapons, or military combatives. There is no need to add "secret moves." My explanations of Heian and Empi kata on this web site did not involve a single extra move. In fact, extra moves would ruin it.
Don't be taken in by this myth.