High-pitch cleavers on surface drives have too much bite- especially the 4 and 5 blades used these days. Off-plane, you just don't have enough bottom-end power to get them spinning to the point that the engines are up to the place in their powerband that they're making enough power to move the boat through the water at a speed where it can break over onto plane. The tubes duct air right to the front side of the top of the prop blades. This lets them slip a bit and bring the RPM's up. Once on plane they do nothing.