Start: Grab a pair of light dumbbells and lie facedown on a bench, letting your arms hang off the sides. Hold the dumbbells with a neutral grip.
Finish: Pinch your shoulder blades together as you use your rear delts and upper-back muscles to raise your arms in a wide, arcing motion out to the sides. Hold for a second or two at the top, then stay in control as you lower the weights. Don't rest the weights on the floor.