The bent-over rear lateral raise and the reverse pec deck are the same movement, done at different body angles. Both exercises work the posterior, or rear, deltoid muscle. Which exercise you choose will depend on the equipment you have. For the bent-over raise exercise, you need a set of dumbbells. For the reverse pec deck exercise, you need a pec deck machine.
For the bent-over rear lateral raise, you need a pair of light dumbbells. The exercise is done with your torso leaning forward, as this positions the weight against gravity. Lean forward so your torso is between 90 and 130 degrees with the floor. Shift your weight back on your heels and keep your back straight; don't let it round out or excessively arch. The dumbbells hang toward the floor with your palms facing each other and your arms extended. Lift the weights out to your sides until your arms are about parallel to the floor.
Reverse Pec Deck
The reverse pec deck exercise is similar to the bent-over raise except you are in a seated position and your arms are bent. Dumbbells cannot provide resistance in this position, but an exercise machine can. Sit in a pec deck machine with the pads facing forward. Place the back of your upper arms against the pads and push back until your elbows move past your sides.
The reverse fly is similar to the reverse pec deck, except that your arms are straight, not bent.
The reverse pec deck and the bent-over rear lateral raise work your rear shoulder muscle, also called the posterior deltoid. This is a small muscle on the back of your shoulder. Other muscles in your upper back, including your traps and rhomboids, assist during the exercise. If you don't have access to a pec deck machine, the bent-over rear lateral raise is a good alternative that only requires dumbbells.
The posterior deltoid is a small muscle and doesn't require a lot of weight to work it to fatigue. However, you can use more weight on the reverse pec deck than on the bent-over rear delt raise because your arms are bent, reducing the space between the point of force and the pivot point. This makes the movement easier. You can create the same effect with the bent-over rear lateral raise by bending your elbows at 90 degrees.