The unconscious mind - the powerhouse of can do/can't do - success/failure, is unusually receptive to that positive suggestion in the trance-like state, and translates through to daily life in a powerful and effective way.
When fragmentation of positive intent is negated, when half a mind to change ceases to be the norm, and when the "inner person" and "outer person" (conscious and unconscious) are in harmony and work together, then remarkable progress can be made.
David Bonner also likens this process to the use of an oven. The oven is useless unless you have something to place within it, but more than that, the ingredients must be in exactly the right proportions.
Therein lies the skill, the initial joint work and understanding, together with the rapport between therapist and client that swiftly needs to be and will be put in place.