The encounter starts as a traffic stop, maybe over a broken taillight or an expired registration. Most times it ends with an annoyed driver clutching a ticket and cursing the officer, themselves, their bad luck or maybe all three. But sometimes it doesn't end that neatly. Sometimes the traffic stop turns into a car chase. Which turns into a car crash. Which turns into a foot pursuit. Which turns into a struggle in someone’s yard. Which turns into a police shooting. Things can spiral in moments. And a police officer's split-second decision during that frantic dance can set a nation ablaze.