Dumbbell flyes are considered an "isolation" exercise for building the chest.
When you set the bench to an incline, you can selectively target the upper chest fibers, the part of the chest that many people are lacking the most.
Muscles Worked
The chest muscles (pectoralis major, aka "pecs") are the primary movers. The clavicular portion of the pecs (aka the "upper pecs") are emphasized in particular when the bench is set to an incline. The front deltoids also get significant work. The triceps are involved much less during flyes than they are during dumbbell presses.
Advantages
Being an isolation exercise, flyes allow for greater recruitment of the chest muscles with less involvement from assisting muscle like the triceps. This lets you feel a strong contraction in the chest alone and allows for a strong mind-to-muscle connection.
Flyes are popular among bodybuilders as a way to add more volume to a workout (on top of presses), to build muscle and to achieve a great muscle pump.
Disadvantages
You can't use as much weight on flyes as you can on presses, so flyes are not as effective for maximizing strength.
Flyes also involve less muscle mass, so if efficiency is important to you, or if you can only choose one chest exercise, it would be better to choose a pressing exercise that lets you use more weight and recruits more muscle mass. That's why flyes are often considered a finishing movement and are typically done in addition to presses, not instead of them.
Equipment needed
Dumbbells and an adjustable incline bench
Execution / Proper Form
Set the bench incline to about 45 degrees or slightly lower.
Grab a pair of dumbbells, sit on the bench, plant both your feet firmly on the floor, then lie back on the bench, rocking the dumbbells into the starting position.
Begin with the dumbbells over your chest with your palms facing each other and your elbows slightly bent.
Slowly lower the dumbbells out to the sides in a semicircular motion, keeping your palms facing up and maintaining the bend in your elbows.
Lower until your upper arms are approximately parallel to the floor or where you feel a slight stretch through your pectorals, depending on what your flexibility comfortably allows.
Return through the same path to the starting position, gently touch the weights together at the top and contract your chest muscles (think "squeeze").
Safety and Precautions
Many people have seen photos of videos of professional bodybuilders doing flyes with a deep stretch. That doesn't mean it's a good idea. Do not over-stretch on dumbbell flyes!
This is not a good exercise to go heavy on. The heavier you go, the greater the injury risk is amplified if you make any mistakes in form. As you do gradually progress the weights, putting slightly more bend in your elbows can reduce the risk.
Lower the dumbbells slowly into a gentle stretch at the bottom, never drop them quickly because that can lead to injury.
Progressions (Make It Harder Or Add Intensity)
You can make flyes more intense by slowing your tempo slightly and using the peak contraction technique where you squeeze at the top of every rep.
