The POW Block is an Item that appears in the 6th main Mario Kart game, Mario Kart Wii. Due to how easy it is to dodge this item, the POW Block doesn't reappear in future installments. As in the other Mario games, it causes a devastating earthquake. This item can be obtained from 5th to 9th place.
The POW Block is an attack and recovery Item. When activated, the POW Block (rarest item in Mario Kart Wii, rarer than the Starman, well, not really.) will travel to all players ahead of you. It makes three warning crashes before delivering the final blow, in which the POW Block icon is completely flattened and shrinks into the distance, and all the racers stop, lose their Items if they have any, and spin out, losing all of their speed. It takes them a long time to get up to their top speed afterwards. It is a great recovery item, because all the players in front of you are slowed down.
The POWerful effects of this destructive item can be avoided. One way is to be off the ground when the POW Block unleashes its power. The other way is to shake the Wii Remote (if using Wii Wheel, Wii Remote held sideways or Wii Remote + Nunchuk), or press any direction on the D-Pad (if using Wii Classic Controller or GameCube Controller) at the moment the POW Block strikes. You will still spin out and lose any items that you might have, but you will do a trick and not lose any speed. In N64 Mario Raceway, the POW Block causes all the Piranha Plants on the ground to jump into the air.