The Bent-Over delt raise, also known as the rear lateral raise is an exercise that builds strength and size of the rear delt, a muscle on the backside of the shoulder.
The shoulder muscle consists of three heads, Most people tend to develop their front and middle delts with chest presses and side lateral raises while neglecting the rear head. This limits your shoulder size and strength. Even worse, weak rear delts cause strength imbalances in the shoulder. Overtime, this imbalance causing the shoulders to round up towards your front, giving you bad posture, and greater risk of shoulder injury.
Don't bounce your back up and down. Bouncing will make you use momentum which doesn't sufficiently challenge the rear delt. It will also might increase the risk for injury. Bouncing the back is usually caused by too much weight, so use a lighter weights to prevent that.
Keep your shoulder blades flat. perform the movement with arms perpendicular to torso, putting your arms too much back will engage your back muscles, making the exercise less effective.
Tilt the dumbbells a little in the top. pretend like you “pouring” out the dumbbells, angle your wrists just slightly downward so your thumb is lower than your pinkie.
This exercise is better to be done with a lower weight and higher reps. If the weight is too heavy, your form will suffer and you'll end up working your back instead of the delts.
You can do this exercise sitting down or standing up. The seated version is a bit easier to work and keep a good posture with. It will also be easier on your lower back.
Avoid clanging the dumbbells together at the bottom. This momentarily releases the tension off your shoulders, lower the dumbbells in a slow and controlled manner.