I believe you are referring to the Switch Hand booster. Activating this booster enables you to switch the position of two candies that are next to each other on the board. This is extremely helpful, for example, if you have a Color Bomb or Coloring Candy and a Fish booster that you want to use together, but there is a single candy standing between them. With this tool, you can switch the candy with one of the boosters so both boosters can be combined.
Quick Tip: if you wish to switch a booster with a plain candy, make sure you grab the plain candy first, otherwise the booster will go off unintentionally.