The opposite of amnesia, hypermnesia is an unusually (some say excessively) vivid and detailed memory for certain events.
It can refer to the apparent enhanced qualities of memory under hypnosis. Unfortunately, hypnotic hypermnesia appears to be an illusion: Recall seems to be enhanced, just as many skills can feel enhanced under hypnosis. Memories seem more vivid. However, even if the thing being recalled is true, many of the apparently vivid details can be filled in inaccurately by the subject's mind, similar to a dream.
"Amnesia or hypermnesia for traumatic events" is included in the working criteria for complex PTSD. This refers to the phenomenon of detailed memories playing back over and over in a person's head, even when the person attempts to avoid them. They may take the form of extremely vivid memories, or even hallucinatory flashbacks in which part or all of the memory appears to enter the person's surroundings.
Hypermnesia can also refer to what happens in the case of an eidetic or photographic memory, in which a person has an ability of near-total recall for a situation.
References:
Kihlstrom, John F. Hypnosis and Memory. http://ist-socrates.berkeley.edu/~kihlstrm/hypnosis_L&M2003.htm. University of California, Berkeley. Accessed 2004.
Herman, Judith. Trauma and Recovery: The Aftermath of Violence -- From Domestic Abuse to Political Terror. New York: Basic Books, 1992.