The idea behind hypnosis for the "non-suggestible" is that you are the person who wants to change, and the hypnosis simply makes you unable to focus on any thoughts that would impede that desire for change.
For instance. a person who is able to quit smoking cold turkey is able to summon enough internal strength to stop smoking as soon as they make the decision and not succumb to temptation. A person who effectively loses weight is able to do the same. Most of us, however, struggle because we are conflicted. Hypnosis helps us to resolve that internal conflict so that instead of fighting with ourselves more and more, we are able to tap into that strength so that we can focus on change in the same way that those who can do it on their own can do naturally.