Pick a moment in a scene, then think of a later—or earlier—moment that either matches (bookend) or shows something opposite (mirror).
Oh, interesting! I always thought of bookends as being scenes that show a beginning and ending: Ennis collapsing in an alley at the beginning of their relationship, Ennis collapsing in an alley at the end of their relationship. Mirrors I thought of as being scenes that show something in common, perhaps "reflecting" each other, but not necessarily forming that sort of beginning/ending narrative connection: Ennis catching the watch, Jack dropping the keys.
Whichever way you look at it, you often wind up at the same place, I think.
Mel, your answer to your own challenge sounds right to me! The rhythms do match.
(Bonus question: Does "No, no, not on your fucking life" remind anyone besides me of anything?)
I've got another bookend involving the phone-call scene:
Scene: On the phone, Lureen tells Ennis she knows who he is, and Ennis tells Lureen that he and Jack worked on Brokeback together in the summer of 1963.
Bookend (earlier):