If this all sounds somewhat dubious, it should.
The hypnotists seem to be relying on the well-known placebo effect, in which a fake treatment can (temporarily and under limited circumstances) have a real effect on health. But the placebo effect only works if the patient believes it is effective.
Neither the placebo effect nor hypnosis can "convince the body" that it had undergone gastric banding surgery, chemotherapy, or anything else. The stomach does not have a mind of its own and can't be convinced, bribed, or fooled into doing anything. Lindley knew full well that she had not in fact had the fake surgery; she requested and paid for the procedure. Trying to fool a stomach into eating less makes no more sense than trying to fool nearsighted eyes into thinking they don't need corrective lenses.
So what accounts for the weight loss by Lindley and others who have undergone this procedure (assuming it's true and not just promotional claims)? The patients were paying more attention to what foods they ate, and in what quantities. It's as simple as that. Most people don't pay a lot of attention to what they snack on, and when they are asked to begin keeping a record of what they eat, they tend to eat less and lose weight.