What is the diminished capacity defense of diminished capacity in a murder case?

Appellant now argues that defense counsel should have gone beyond the defense of diminished capacity so vigorously and thoroughly presented at the guilt phase and should have requested instructions on "imperfect" self-defense that is, the honest, but unreasonable belief that one is in danger which might negate the malice aforethought necessary for a murder conviction, thus reducing the offense to manslaughter. (People v. Flannel (1980) 25 Cal.3d 668, 160 Cal.Rptr. 84, 603 P.2d 1.)