At the time there was no one training skills that would be useful in communication that involved behavioral genetics, covert conditioning, subliminal technology using social psychology and neuropsychology as a foundation for this field.
NLP and hypnosis were essentially closed fields that were tangential but didn't really overlap with Covert Hypnosis.
Traditional Hypnosis could be isolated to the four basic categories.
1) Stage Hypnosis relies on role playing behavior.
2) Ericksonian Hypnosis revolves around the potent use of story (also significant in Covert Hypnosis).
3) Hypnotherapy which draws from an evolved psychoanaltyic approach. and finally
4) Direct Suggestion Hypnosis.
Erickson's work had taken some strange turns into "hypnotic language" which certainly has a place in subtle influence but the effectiveness of Ericksonian Hypnosis was far more about story and strategic listening than "language patterns."
NLP expanded for awhile in the late 80's and early 90's at which time the field essentially closed. No real research of substance was done to improve the technology thus rendering it less valuable as time went on. Nevertheless there were many useful behavioral change devices that come from NLP.
What was missing was a scientifically derived set of influence factors that could be taught and learned. Many of these factors would involve art. All would involve science. What worked would be kept. What didn't would be given up.
As time went on I continued to grow the study of Covert Hypnosis but many others took the rather captivating term and imagined that it would be a neat way of describing hypnotic language patterns and similar influence approaches that generally were not tested, proven or shown to be effective.
Covert Hypnosis originally struck me as combining behavioral genetics with subliminal technology which was just coming into it's own in the late 90's and psychological principles both social and individual which included a dash of the hypnotic but not as most think.
Really what drives Covert Hypnosis is the discovery and then application of influential factors from the environment, the context, from the individuals own expectations, and dealing with resistance and reactance.
Since then it has been studied at University under titles like "Covert Persuasion" yet another term I coined in the early 2000's and then wrote the first book and did the first research for.
That is a story for another day.
Covert Hypnosis is best noted as subtle but overarching influence. It is more science than art but there is art in effective application of often very large but unseen factors.