Tension is the stimulus for growth. Muscles experience the greatest amount of tension where the resistance curve is at its peak in the concentric point of an exercise. And tension can be manipulated in order to spur new mass gains.
One way to do that is to raise the resistance curve at the point where it starts to decline so that the muscles are at a disadvantage for a longer period of time. This is where accommodating resistance comes into play, like using bands to increase the resistance where it normally would descend.
Here's one way to add accommodating resistance to flyes.
Wrap the band around your back, or even below the bench (all depending on the strength of the band), to keep tension more constant through the movement. Now, instead of tension "falling off" at the top of the movement, it'll be at its peak.