Buffy the Vampire Slayer - TV Tropes: Wham Line - Part 2 (Part 1)
Wham Line: a Wham Line is a line of dialogue that radically alters a scene. A scene is headed in one direction, then the line is uttered. Afterwards, the scene is going somewhere very, very different. For it to be effective the audience must not see it coming — not just not knowing the exact information, but not expecting any kind of surprising, or even significant, line there at all.