There is some rationale to it. Assuming you aren't in the insta-crispy zone and you don't take a gamma rays shower, duck-and-cover might well protect you from things like windows shattering due to overpressure.
And flash burns, which can be pretty nasty.




Incidentally, that's actually the first time I'd seen that test card accompanied with a female speaking clock, presumably added to try and freak out the most hardened of souls in the small hours of the night.
