As a metaphor of what is meant here, consider what some people call an “earworm.” That is a song that you cannot get out of your head. Nobody knows why that happens for sure, but it could be that the mind is looking for completeness, and you only know part of the song. So the mind gets stuck in a loop trying to force you to remember it. Some say that actually singing the song in its entirety may keep the annoying fragment from replaying itself over and over. So traumatic memories may work the same way. There might be a bit of responsibility for the original event that you are refusing to take, or you are unable to look at the large picture. So your mind is forcing you to relive the trauma until you can find the missing pieces, and hypnosis might allow you to drop your guard on the issue long enough to face the event in its entirety and thus resolve it.
Can hypnosis help you forget a horrible memory
Not really, though it may help you to resolve the memory. Perhaps there were aspects of the memory you are not willing to remember. Maybe you have buried guilt and self-blame for what happened. Or maybe the experience was just too traumatic to want to remember. So maybe hypnosis could get you to overcome your reluctance to face what happened for what it was.