The pullover is an old time classic exercise that was Favorited by many famous bodybuilders in the past. The pullover hits your entire upper body, it works on your upper chest, the serratus, intercostals, abs, lats and triceps.
This is a unique exercise because it works on opposite muscles at the same time - the chest and the back. It is considered more of a chest exercise, since study showed that the pullover stimulates the pectoralis major of the chest more than the latissimus dorsi of the back.
Another benefit of the pullover, is that it works your chest from a whole new different angle, compared to the usual bench press or chest flyers. This will stimulate different muscle fibers in a whole different way, which can spur on new muscle growth in the upper chest that you didn't trained before.
One important thing to know, is that this exercise might not fit everyone, since it may cause shoulder problems for some people.
This exercise puts your arms in a bit of a weak position, so be sure to start out with a light weight, and gradually increase the resistance as your body adapts over time.
If you feel pain in your shoulders doing this exercise, you can try using two dumbbells, one in each hand instead of one for both. this will cause less stress on the joints.
Don't stretch your abs too much when the dumbbell is lowered behind your head. This might stretch the linea alba which is the connective tissue in the middle of your abs, causing weakening and even tears over time.