What is the test for instructing a jury on a defense in a criminal case?

" ' "It is settled that in criminal cases, even in the absence of a request, a trial court must instruct on general principles of law relevant to the issues raised by the evidence" ' and ' "necessary for the jury's understanding of the case." ' [Citations.] It is also well settled that this duty to instruct extends to defenses 'if it appears . . . the defendant is relying on such a defense, or if there is substantial evidence supportive of such a defense and the defense is not inconsistent with the defendant's theory of the case.' " (People v. Brooks (2017) 3 Cal.5th 1, 73.)