It has long been a practice to slit banners as a counter-measure to the pressure of wind on the banner surface. These are usually semi-circular cuts in the substrate.
However, not everyone agrees that this is an effective practice. The chief criticisms are that they detract from the aesthetics of the banner, they weaken the banner material and they don’t do much to lessen the impact of wind pressure. Apparently, slits only reduce the wind pressure by 10 to 15%.
An alternative to slits is a heavier weight banner material and better installation methods. For instance, weave the rope through the grommets and tension and tie the rope to keep the banner taught. A banner that flaps tends to tear. Another method is to secure the banner with bungee cords. It’s said to work well by stretching and preventing the wind from tearing the material.