Tl; dr: it's about adding a second layer to your communication which attends to the subconscious, not unlike art. It was originally for therapy, but unfortunately a lot of businessdorks in the 90s got into it and perverted it.
He said after many years he wasn’t sure what hypnotism was exactly, or even if it was an identifiable thing at all, and that in a lot of ways he was just giving people license and cover to do stuff they probably wanted to do anyway. You can’t hypnotise people to do something they don’t want to, apparently.
So if he says “Come up on stage and cluck around like a chicken, make a real show of yourself in front of the crowd”, then quite a few people will go and do it and come away saying “That wasn’t me, the hypnotist made me do it, but what laugh eh?”.
He was less sure how this might apply to (for example) hypnotic pain control, but it was an interesting take.