When you are scared and facing fear, your body believes you are in danger and something is threatening you so putting your hand on your heart and reassuring yourself that you are not in harms way can instantly relieve any pressure you might be feeling.
I did this on Saturday before I got on stage. I did this right before wearing a bikini for the first time. I do this every time I get on a plane and I did this on Monday right before I released the new Scarred Not Scared video.