Part of a hypnotic induction - the part where you are being told to relax deeper and deeper - is a way to eliminate tensions from your body. When you do this and relax deeply, the parts of your brain which keep track of all your tensions and concerns have less to keep track of.
That in turn brings your brainwaves down, hopefully going from an awake and focused beta state into a relaxed alpha state. When you do this the portals to your subconscious mind open up and you increase your states of suggestibility. In that relaxed state you can then be made to easily accept new “programs” for yourself.