Mr. Kerker was actually using the kata for his applications, but the kata sections can be defined in infinite ways, as pieces of what you think of techniques, to the standard way of looking at the techniques to groups of techniques.

Technique is of course technique, the system of study at some point is less relevant (except in the way they gather and discharge energy for the technique useage).

The biggest key is as the study progresses the power behind the attacks becomes more serious and the power of the counter attack likewise keeps stepping up. Not to the point your partner is truly injured, but to the point you're having to deal with a truly focused attack, and you have to respond and release serious energy into the attacker (as in striking or kicking or locking or projecting).

All of which could be accompanied with makiwara study, etc.

His capabilities came from the 40+ years of work his instructor did into Isshinryu's potential, and 15 years of hard contact training to impart his skills.

The problem with most books and clinics is they gloss over the real path, the long time requird to build skills. A clinic too often has too many individuals that require softer training and slower study, so only a glimpse of what potential exists can be shown. And even if the instructor has skilll, and demonstrates it against any attack, eveyone leaves the clinic with almost nothing, becasue the key ingredient is you have to spend years of hard training with that instructor to get it.

Of course you can do it on your own, just as Mr. Harrill did, spending year after year, working on your students (and equally giving them full chance to work on you) and work it out yourself.

Or you could move to Carson City, Iowa, approach Mr. Kerker and if he agrees to take you on as a student, you can get it quicker than 40 years, but it doesn't happen overnight either.

The methods you mention, from what I've seen, do nothing for me. And there isn't just one layer to this training, or one right answer for everyone.

Just choices based on working very hard at whatever level you pursue.
victor smith bushi no te isshinryu offering free instruction for 30 years