This exercise primarily works your pectoralis major muscles (your chest). Movement is performed with your back lying on a bench.
1) Lie flat on a bench with a dumbbell in each hand. Rest the dumbbell on your thighs, then use your thighs to help raise the dumbbell.
2) Raise the dumbbell like you're pressing them, but stop before locking out. This is your starting position.
3) Slightly bend your elbows, and lower your arms out at both sides in a wide arc until you feel a stretch across your chest. Return to starting position.
Here's a visual:
If you have any additional tips for this exercise, please let me know in the comments below and I will add them to this card. :)