The POW Block is an exceptionally rare Item in the Super Mario Bros. series. Creating a huge bang, these POWerful (get it?) blocks can take out all enemies on screen and even bring down a shower of Coins.
Similar to its role in other games, a POW Block, once activated by a player, will let off warning shock waves to all the racers in front of the user (similar to the Blooper). On the 3rd strike, it will smash the ground and spin out all the racers who were effected by it. POW Blocks also take away any Items the victims had equipped or in storage, as well as their speed. This makes it a great recovery Item, as it damages all the drivers who are ahead.
There's a few methods of avoiding the devastating effects of this dangerous item; anytime you are in the air, whether doing a Trick, blasting through a cannon, or just a freefall, you aren't touching the ground. Which is good, considering that's where the shockwave came from...
However, if you are on a course with almost no jump pads or ramps like Luigi Circuit, you can shake the Wii Remote. This supposedly makes you "hop" a little, and when the quake comes, you will spin around but won't lose any speed. Blue flashes will be emulating from your Kart. Unfortunately, you will lose any Items you have garnered, so use them right away before the third strike occurs. Otherwise it wouldn't hurt you at all, and Mario Kart is all about unfairness! So as you might guess, the POW Block is more useful on CPU opponents, as they can't evade the shock wave.