The POW Block didn't even do much of anything. You could always avoid it by shaking the controller, and you didn't lose much speed or anything (although maybe you lost your item? I don't remember)
What REALLY sucked was that terrible thundercloud item. The one that gave you a speed boost, but then shrunk you after a certain period of time. The idea was that you're supposed to crash into another player and pass it on, but that's hard as hell to do. Plus, then you lose your speed boost. Forget Blue Shells, those were the most annoying item in MKWii for me by FAR.
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You have no time to ponder such questions, as Link has just grabbed a Smash Ball.
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