There is a saying that the hypnotist has the power and ability to resolve challenges faced by his client. Understanding this, when a client visits a hypnotherapy clinic, she trusts and believes that the hypnotist has the professional knowledge to help her with her issue.
Because of the trust on the hypnotist, the client focuses her attention on what the hypnotist says and excludes everything else. Such focused or narrowed attention is what produces the desired effect for the client who undergoes the hypnosis.