The POW Block is an Item that appears in 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 crushed, 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.
Two defenses are possible against the destructive item. One is to be in the air while the POW Block comes down. The other way is too shake the Wii Wheel, Wii remote, or press the D-Pad. You will still spin out and lose any items that you might have, but you will will do a trick in mid air and not lose any speed.