Start: Grasp two dumbbells and lie faceup on a flat bench, sliding your hips completely off of it so the edge of the bench hits the middle of your lower back. Keep your core tight and squeeze your glutes to stabilize your hips.
Execution: Begin with both weights pressed to full-arm extension over your chest. Lower one dumbbell toward your outer pec until your upper arm is roughly perpendicular to your body, then press back up. Lower the other weight in the same fashion, always keeping one weight up. Don’t allow your hips to drop or rotate during the movement.