The dumbbell pullover is primarily going to work the upper back muscles. This exercise also works just about all the muscles of the upper body to some degree. The largest muscle in the back is scientifically known as the latissimus dorsi. You might see it referred to as the "lats" or "wings." The lats are used any time you pull something. For example, when you open a fridge or a car door, you are primarily using your lat muscles. Instructions for the dumbbell pullover are listed below:

First, select the appropriate dumbbell and set it near the edge of a weight bench. Position your upper back (shoulder blades) on the bench. Reach over, grasp the dumbbell, and hold it over your head with an overlapping hand grip. Next, keep your arms locked into position, and lower the dumbbell straight back until it is parallel to the floor. Return to the starting point. Repeat the movement as needed.

IMPORTANT: Your elbows can be slightly bent, but they must remain locked into position during the pullover. The goal is to try to minimize activity of the triceps. Be sure to keep your chest high at all times.