This exercise is used to pack on mass to the chest and also gain strength in the chest, shoulders, and triceps. Using dumbbells rather than a barbell uses more stabilizer muscles.
Begin by sitting on a bench with a dumbbell in each hand, the dumbbell should be resting on each of your thighs. Lie back onto the bench and kick the dumbbells up to your shoulders, one arm at a time. Rotate your wrists to get the dumbbells into the proper position; the dumbbell should be just outside of your chest. Begin the movement by pushing the dumbbells away from your body using your chest muscles. Lock your arm at the top of the movement, but do not touch the dumbbells together. This will take the tension off of your chest muscles. Finally, lower the dumbbells back to the starting position and repeat the movement for a set number of repetitions.
A spotter may be needed to help you get the first repetition off your chest. It will be much harder for most people to start with the weight on their chest. Make sure to keep your entire body tight as you do this, firmly plant your heels into the ground and do not let them move.