Many combative secrets to classical systems are taught through 'kuden', or oral tradition. Others you can only get through feel--they don't always make sense verbally. Ways of attacking multiple balance points, how to use footwork to move closer to an opponent without seeming closer, how to hide the true length of a blade through taking advantage of 'blind spots' in the eye...etc, these things are taught from the beginning to new people, but very few can discern this for years. Ironically, alot of scroll 'secrets' in koryu systems have very little value to today's society--medicinal salves for wounds(antiquated stuff if you have access to a drugstore or are good with herbs), mixtures to keep lice out of yoroi(armor)--not very useful in our time period. I've never seen RVD's stuff, so I cannot comment on its quality. I will say that video can be useful if you have a good understanding of movement. I have old footage of backyard training and various seminars that are non-commercial material. There are often little jewels for training interspersed throughout, but it takes a good grounding in taijutsu principles to pick up.