In true supervillain fashion, Jeanine decides to execute Tris through the incredibly inefficient method of slowly drowning her. Sure, it's scary, but it's so slow. The Erudite are studying this for some reason.
But luckily, Tris's mom shows up and rescues her. Mom gets her out of there—and reveals that she herself was Divergent. (So does it run in the family, like red-hair or love of Buffy the Vampire Slayer?)
But while they're running away, some Dauntless almost catch them. That's when Tris's mom sacrifices herself so that Tris can get away and find dad and bro (who are hidden in a nearby basement).