Growth is stimulated largely by muscular tension. Muscles experience the greatest amount of tension where the resistance curve is at its peak in the concentric point of an exercise. But tension can be manipulated in order to spur new mass gains. Here's an example:
Just anchor the band with a heavy dumbbell at your feet. Bands around your neck will also naturally bend you forwards and cause the pecs to get into a very deep stretch. They'll also create more tension at the top of the rep.