Just as a Wham! Episode is an episode that radically alters the Story Arc, a Wham! Line is a line of dialogue that radically alters a scene. A scene is set up, plays out as expected, a line totally at odds with the established tone of the scene is uttered, and that line changes the course of the scene dramatically from then on out. The result shocks the audience out of their current mindset, and engages them fresh in the work.
The audience must not see the moment coming -- no Foreshadowing, no Foregone Conclusions, no contextual clues of any kind. This is a key component of the Wham Line; if the audience is led to believe that something big is coming, no one will be surprised when the big thing happens -- in other words, there will be no "Wham!" (An expected surprise which subverts your expectations as to the nature of the surprise is just that: a Subversion.)