San Francisco is a great slam prevention system that once I started using it, I couldn't give up. All of my regular partners use it to some extent. I can't describe it better than what Mich Ravera has said about it, so just download the PDF and become initiated!
The PDF covers the San Francisco (also known as "Warren") convention, which asks for aces and kings simultaneously using "total controls" (Aces=2, Kings=1). With a single ask-and-answer, you can determine if slam is viable or if you're missing critical controls. The guide includes normal, strong-side, and weak-side responses, plus follow-up bids, exclusion inquiries, and the related "San Francisquito" method for asking about controls in a specific suit.