When we react we are not in the present moment. We are re-acting/acting again/back or against something. As Albert Einstein said, "Insanity is doing the same thing over and over again and expecting different results." We react when we're triggered, and very often it's a knee jerk reaction where we return to an old conditioned state which needs healing and resolving.
The first step is to be aware. When you're mindful you notice, in the moment, that you are re-acting, which is when you move into responding instead. This brings you immediately into the present moment, where you can act from a position of clarity and authentic power.