John Ryder Ph.D who is a psychologist, hypnotherapist, and the author of Positive Directions in the article "Does Hypnosis Improve Memory?" states that hypnosis can increase focus and depending to the skills the hypnotherapist has, can help people remember.
Ryder states that a memory is still not fully understood on how it works. Problems can be formed in both types of memory we have- the short term and the long term either in retrieval or storage.
Hypnosis' ability is to heighten focus in limited area, slow down the process, and let the imagination go without any distraction. Sequence of details of where that memory is stored particularly by the patient is brought out by the hypnosis.
Details could come out from such concentration of the experience and has been useful in the field of society where witnesses were called in to recall the crime scene in detail.
Trance allows the hypnotherapist to go deeper in to the patients memory and thus by leading them in to their vivid memories, help them recall the lost/ forgotten memory. This also digs in to the subconscious or unconscious mind that are not in the normal comfort zone of many people.
I have personally never done hypnotherapy but after reading this article made me want to try. Also, long time ago in my school, and hypnotherapist had come in and had hypnotized 10 students in front of the whole school and ordered them to perform such actions. I then began wondering what part of the memory and brain the hypnotherapist control in order for the person to actually physically move without noticing.