Any person using force intended or likely to cause death or great bodily injury within his or her residence shall be presumed to have held a reasonable fear of imminent peril of death or great bodily injury to self, family, or a member of the household when that force is used against another person, not a member of the family or household, who unlawfully and forcibly enters or has unlawfully and forcibly entered the residence and the person using the force knew or had reason to believe that an unlawful and forcible entry occurred.
As used in this section, great bodily injury means a significant or substantial physical injury.Effectively, if a stranger forces way into your home, the presumption is that they intend a resident great bodily injury. If you use deadly force against someone forcing entry into your home, this presumption puts you at great legal advantage if the prosecutor decides to pursue criminal charges against you.
I can remember when California passed this "castle doctrine" law. My recollection is that the motivation for passage of this law was a case out of Los Angeles in 1982 or 1983. A burglar forced entry into someone's home, and the woman of the house either stabbed or shot the burglar--and the district attorney prosecuted her, because she didn't have proof that the burglar intended her death or great bodily injury.
If this seems utterly bizarre: actually, it is not. There are a lot of people out there who are so horrified by deadly force that they are prepared to make the most absurd excuses. A co-worker, many years ago, explained that there are all sorts of perfectly innocent reasons why someone might force entry into your home. "Maybe there's been a traffic accident, and they need medical supplies." (Not a dummy; he's now a multimillionaire.)
Anyway, I have seen the case that caused the legislature to pass 198.5, but I can't find it now. If you know the details, please let me know.