The dip provides a way to develop the upper body pushing muscles, but carries a major safety risk.
The dip focuses the chest, front half of the shoulder, and the back of the arms. These areas include muscles of the sternal and clavicular portions of the pectoralis major, anterior deltoid, and triceps brachii. Although not mentioned in most sources, the front half of the middle deltoid receives intense work as well as long as you lean forward enough. Major joint motions include arm extension, shoulder flexion, horizontal arm adduction, and scapula abduction or protraction.
Muscles in the upper back, shoulders, and torso stabilize. Since depression of the shoulder occurs, the lats also contract intensely. As a closed-chain movement, the core works harder versus an open-chain movement such as a bench press since your body moves through space. This guide focuses on dipping with parallel bars.
You will need parallel bars and a dipping belt with weight plates. The safety feature could come from the floor or a step. Dip stations have steps to each side that make it easier to setup your position. Using dumbbells or weight plates pinched between your legs may work initially, but this requires a low enough surface to stand on before you begin or a partner to hand off the weight. Regardless of setup, when your knees extend, you should feel able to stand on a surface to end the exercise, just below the normal range of motion with the arms parallel with the floor with the elbows only slightly higher than the shoulders. You must therefore bend your knees during the exercise to avoid grazing the floor.
A lot of advice on the dip comes from bodybuilding. For example, some recommend using an upright posture to focus on developing the triceps, while ignoring that this position harms the shoulder terribly. Instead, you should lean forward not just to fully involve all the upper body pushing muscles, but to protect the shoulder by attempting to match movement as if lifting in a horizontal plane.
Due to the depression of the shoulder, this exercise carries a great risk to the shoulder due to the great shearing force it creates. Some trainees can lean forward enough and limit range of motion to prevent this effect, but some shear will occur no matter what you do. I suggest the bench press foremost and possibly a weighted push-up before resorting to the dip. If you can perform this exercise safely without any shoulder pain though, then it can serve as an excellent option.
Pull the shoulder blades back and keep the chest out. Look down and tuck your chin against your chest. Staring down will encourage you to lean forward. Use a parallel grip with the palms facing inward. You should feel the bar near the heel of your palms as opposed to near the fingers. This prevents too much wrist extension and uses your strength efficiently. Most trainees feel better with wider diameter bars versus narrow bars. Having a large bar for your hands to wrap around seems to protect the joints better due to better mechanics.
Bend your knees and lift you heels toward your butt. Lock out your elbows fully so that your arms support your bodyweight. You may need a box, bench, or step depending on how low the floor is after you elevate even if using a dip station.
Avoid the two extremes of shoulder position, with either pure horizontal adduction or pure flexion. This means do not flare or tuck the elbows. Neither extreme works well. The arms should place at about a 45° relative to the body. Some recommend a slight elbow correction by rotating the arm internally. Although this aims to keep the shoulders healthy, it actually feels unnatural and can stress the elbow.
Lowering (Negative) Phase:
Descend until the elbows slightly surpass the shoulders or just slightly more than 90° at the elbows. Continue to lean forward and keep your head down. Stay tight and squeeze the handles. Keep the forearms roughly perpendicular to the floor at the bottom position. Try to keep your wrists aligned with your shoulders.
Lifting (Positive) Phase:
Push back up while maintaining the downward stare. Pause when you reach the top for a moment. With heavier weight, the top will feel somewhat less stable, and going too quickly may cause you to lose your balance.
Stand on the surface to end the set.
The dip can serve as an excellent push for the right trainee. For most, the risk to the shoulder remains too great. If it feels right though and you can lean forward sufficiently, then it can certainly work and will train many muscle groups intensely.