Purpose of Exercise: To develop the mass of the pectorals. The function of the pectoral is basically to pull the arms and shoulders inward across the body, and this is exactly what you do using a bumbbell fly movement.
Lie on a bench holding dumbbells at arm's length above you, palms facing each other. Lower the weights out and down to either side in a wide arc as far as you can. The palms should remain facing each other throughout the movement. Bend the arms slightly as you do the movement to reduce the stress on the elbows. Bring the weights to a complete stop at a point in line with the bench, your pectorals should be stretched as much as possible. Then lift the dumbbells back up along the same wide arc, as if giving someone a big bear hug or trying to hold a barrel. When at the top squeeze the pectorals together to finish the movement.
Insider's Tip: Do not press the weight, let the pectorals bear the weight and bring them up.