Here's exactly what makes it work:
- The tagline uses the prohibitive mood (no, not "Watch out, kids, I'm in a prohibitive mood tonight!"--I mean the grammatical one);
- The tagline echoes several pop culture phrases and memes; and
- The ad forces the reader to make a cognitive connection between the image and the copy, unifying them into a greater whole.
"Don't Be in a Prohibitive Mood" Is In the Prohibitive Mood
The imperative mood is a grammatical feature of verbs that command: verbs conjugated in this mood (such as "stop doing that," "write it now," "drink up," etc.) command an action.
The pairing of an imperative with a negative modifier (adverb or adjective) is called the prohibitive mood: "do not," for example. Using a superlative (a word describing the utmost degree of something: best, last, most, least, etc.) like "never" makes this command form extremely strong...well, actually, the strongest it can be. So "never underestimate" is superlatively prohibitive--and very strong.
Never Underestimate the Power of a Pissed Off Woman
"Never underestimate" is a bit of pop culture, too, starting popular phrases like this one
and many others, from FDR ("never underestimate a man who overestimates himself") to rampant internet memes like this one. So it's familiar and it flows off the tongue, and it helps to transmit broader cultural ideas* which in this case are also related to imperatives ("never, always," etc.), helping to underscore the message that the brand is putting across, the "we want you to do X" message, i.e., "buy our toast."
This ad is synergistic: it's more than the sum of its parts. The image doesn't work without the tagline; the tagline doesn't work without the image, but together, they're a thing of beauty. The image grabs you ("ha ha! made ya look!") and the tagline provides the payoff--and makes you look even closer.
Brand? What Brand?
It's a great ad, with great copy that really works well. And certainly you'll walk away from it with the concept that toast is more than just a vehicle for jam. But will you remember which brand of toast is so much better? That remains to be seen...
*Wikipedia actually has a pretty interesting discussion of memes; see especially Reference 1.