Low to high cable flyes perfectly mimic the line of pull and action of the clavicular pectoralis. It's one of the best exercises for "filling in" the upper chest up near the collarbone.
Start with two pulleys set in the bottom position and have your palms facing forward. Your upper arms (humerus) should be at about a 30 degree angle away from your sides.
Using your upper chest to pull your arms up and in, raise the handles up and together so that they come together near mouth level. The path of the cables will draw an upside-down V.