Lie on a slightly inclined bench so that your feet are planted firmly on the ground behind you.
Straighten your arms out behind you as if you are pretending to be superman.
Then bend your arms forward at the elbows and bring the dumbbells all the way up.
Slowly return them to the straight back position again.
There are two keys for you to really benefit from this exercise. The first is making sure you get your arms back as far as possible, don't just come halfway back or you will deprive yourself of the most beneficial part of the workout.
The second key is to make sure to go slowly through the movement.