Abreaction happens primarily in trauma survivors. It is a phenomenon which seems to be a powerful part of healing, although, if not handled well, it may re-traumatize. This is an unpleasant response that can occur in any therapy interaction. It may occur at any time in hypnosis due to accessing areas in the mind beyond conscious awareness and unwittingly triggering uncomfortable memories or feelings which are then brought to conscious awareness. Patients may become very distressed within moments of this occurring, and it can be a response to a real or an imagined experience. A patient may scream, shout, sob, sweat, shake violently, curl up into a foetus, gag, it can be a truly unnerving experience for the unwary! On the rare occasions that this occurs, and if the patient comes out of trance, it may be useful to take the patient straight back into the hypnotic state to resolve the difficulty or to access calm. symptoms still alleviated. In the vast majority of cases abreaction does not occur and the therapist is left trying to help the patient uncover the emotional links to past events which have led to the present symptom complex.

https://youtu.be/zdXLvJHlPPQ  Hypnosis course Link