This exercise will not only work on your upper chest but it will also work on expanding your rib cage which will in turn make your chest look fuller, wider and stronger. This exercise is one of the most overlooked exercises for the chest.
It's important to note that if you're a beginner you might want to start out with a lighter dumbbell. Once you get the hang of executing the movement you can go as heavy as you want.
For those of you that are already familiar with this movement you can go super heavy but pay attention to your neck. It's important to set yourself up right on the bench to aviod pulling a neck muscle. Thus, take your time and get nice and comfortable on the flat bench.
Another item worth mention is that the ideal situation would be to have a partner help you with lifting the dumbbell and putting it back down once you are done with the movement. If you do this alone you run the risk of hyperextending your (upper) back or shoulder muscles. If you don't have a training partner don't be shy to ask a trainer at the gym to give you a hand.