If you're opening the case and the top doesn't come off easily, don't force it. Forcing a computer case may damage the box and its contents. Granted, the plastic envelopes that encase computer components (and other modern electronic products) challenge the most avid box shredder. Nonetheless, force will merely strengthen the case against you... so to speak.
I pondered what that might mean for a while and finally gave up. Any idea as to what exactly the author means by plastic envelops and avid box shredder? What does all that have to do with shredding cases made of plastic? I don't understand the joke.