§ 1162 Jury to try sanity.

If a judge has doubts about whether a defendant is mentally competent, either before trial starts or after conviction but before sentencing, the judge has to stop and deal with this question first. The judge does this by putting together a jury just to look into the defendant's mental state. This jury can be made up of people already called in for jury duty that term, or the judge can have new jurors summoned specifically for this purpose.

The whole point is to figure out if the defendant is sane enough to go through the legal process. This matters because someone who isn't mentally competent can't properly understand the charges against them or help with their own defense. So before the case moves forward, whether that's starting the trial or handing down a sentence after a conviction, this separate jury gets to weigh in on the defendant's mental fitness.