Using suggestion, the therapist offers alternatives to fatty or rich foods. Using metaphors, he'll encourage you to eat better, and often less.
More specifically, urges for sugary or fatty foods (depending on an individual's weak points) diminish, hunger fades and trips to the supermarket stop being quite as stressful! You'll find yourself naturally reaching for healthy food, and a fennel salad will really get your mouth watering like pizza used to.
Note: if you're a cheese fiend, hypnosis won't put you off it for life: it will simply regulate your cravings!