The POW item is a returning item from previous Mario Kart installments. It can be used to shrink all nearby opposing racers, making them easy pickings for a quick item steal or a swift bump. Getting hit by a POW while using a Starman will cause you to lose your temporary invincibility.
To avoid getting hit by a POW, the best thing to do is to stay away from other racers as much as possible. If you see someone with a POW, try to get behind them or to the side so that they can’t hit you with it. You can also use items like mushrooms or stars to give yourself a speed boost and get away from the POW.