By changing the angles you can greatly increase or decrease the difficulty of a cable fly.
I begin with a standard high to low cable fly with a slight forward lean. This is the most common variation. I use heavier weight here which requires a greater bend in the elbow to increase lever strength.
Next is a fly done with my chest horizontal to the floor. Slightly harder but allows incredible isolation of the chest.
Third is a cable cross-over. You should notice the angle at my elbows has increased which increases the difficulty. This removes some of the help from my arms and shoulders which requires me to drop the weight. My goal is to cross my arms in front of me while totally isolating my chest with a pause at the halfway point of each rep. I alternate which arms crosses on top each time.
As I finisher I rotate my pinkies inward as I finish the concentric portion of each rep. This forces a greater contraction in the lower portion of the chest.