This exercise strengthens the tricep muscle, which is primarily responsible for extending (or straightening) the elbow.
Begin standing with your feet hip-width apart, holding a weight in each hand with your palms facing each other. Lift your arms so that your arms are straight up over your head with weights pointed toward the ceiling.
Slowly bend your elbows and lower the weights behind your head, keeping your upper arms still. Straighten your arms to return to the starting position. Repeat this movement.