For this to work you must have drifting set to manual. Before the third strike from the Pow item, start to drift so that your bike will be in the air for the third strike. When you land you should spin only once and maintain about the same speed.
The moment the POW block has plundered its third strike, immediately pop a wheelie. You will not actually go into a wheelie, but the POW does not affect you and you will continue at almost the same speed. You will know that it worked when you see about four blue waves appear from underneath your kart.
If you are using the automatic transmission during a race, lift the Wii Wheel quickly just as the third blast of the POW goes off. You should only spin once and not lose any speed.
To avoid the effects of the POW block item, as soon as you see the POW icon begin shaking the Wii Wheel as much as possible. You can go unaffected by the POW with this technique. It is easier to shake the Wii Wheel and maintain control if you are in a powerslide before you start shaking.