Being in this moment slows down and deepens your breathing which clears your mind and lets you notice important structural points of the piece.
Being in this moment focuses your mind which helps you to stay in the current measure you are playing to prevent mistakes.
Being in this moment carries your thoughts from the previous and future sections of the piece back to where you are right now which makes your performance calm and peaceful.
Being in this moment eliminates performance anxiety which lets you feel great when you play.
Being in this moment is not easy to achieve because of all distractions you are facing during the performance but at the same time it's a simple concept.
All you need to do is to come back to now. Gently but persistently.