How does the defense of duress work?

The defense of duress "negates an element of the crime charged--the intent or capacity to commit the crime--and the defendant need raise only a reasonable doubt that he acted in the exercise of his free will. [Citation.]" (People v. Heath (1989) 207 Cal.App.3d 892, 900, 255 Cal.Rptr. 120 (Heath ).) To establish the defense, the defendant must show he acted under such immediate threat or menace that he reasonably believed his life would be endangered if he refused. (Ibid.)