Posthypnotic amnesia is like having an invisible magic eraser. It is when someone who is hypnotized can no longer remember things that happened to them while they were hypnotized. When the person wakes up from being hypnotized, they will forget the things that were said, the instructions that were given, and any memories that were brought up. Hypnotists sometimes use post-hypnotic amnesia to help people forget painful memories, like traumatic events.