The laying lateral raise in a great exercise for your shoulders. This exercise mainly work posterior deltoids (rear shoulder) and lateral deltoid (side shoulder). The front delts and the traps also assist the movement. This exercise will help you to builder bigger, and wider shoulders.
The shoulder muscle consists of three heads, Most people tend to develop their front and middle delts with chest and shoulder presses 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.
This is a similar movement to the bent over rear delt raise. Using a bench will help you to keep your torso stable and ease the pressure on your spine and lower back. This will result in a greater isolation of the rear delt muscle.
Use a lighter weight. This exercise works best with low weight and a higher rep range (around 10-15 reps per set). Using weight that are too heavy might compromise your form, and you will end up working your back, not the delts.
You can do this exercise on a flat or an incline bench. The flat bench will target the rear delts, while a slightly incline bench will incorporate the side delts as well.
Keep your arms straight throughout the movement. Allow only a slight bend at the elbow. If you find keeping your arms straight too difficult, it is usually a sign you are using too much weight.
Keep your chest on the bench. Avoid the temptation to raise your torso as you lift the weights, this will take the pressure off your shoulder to other muscles, making the exercise less effective.
Lower the weight slowly. Don't just drop the dumbbells down, but lower them in a slow, controlled manner. Lowering them slowly will keep the tension on your delts, leading to more muscle growth. It will also prevent you from using momentum.
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.