The dash really has nothing to do with the "if". Somewhat like a colon, a dash designates a strong relationship between what preceeds it and what follows. Or, much more eloquently, in the words of Sheridan Baker (the emphases are mine):
Originally Posted by phoenixqn81
To come to grips with brevity, the best way is to read poetry, perhaps the lyrics of pop songs, and to figure out each text for yourself.
The dash says aloud what the parenthesis whispers. Both enclose interruptions too extravagant for a pair of commas to hold. The dash is the more useful—
since whispering tends to annoy—and will remain useful only if not overused. It can serve as a conversational colon. If can set off a concluding phrase
It can insert a full sentence—a clause is really an incorporated sentence—directly to a key word. The dash allows you to insert—with a kind of shout!—an occasional exclamation. You may even insert—and who would blame you?—an occasional question. The dash affords a structural complexity with all the tone and alacrity of talk.
S. Baker, The Complete Stylist and Handbook, 3rd ed. New York: Harper & Row, 1984, p. 403.