You have probably played with a postcard or index card, gliding it on ground effect across a smooth desktop or glass table. Amazing how well they go. Especially if the front edge is bent up a bit to gather air pressure. Plus you can concave the card just slightly. Then put a load on it like a coin. The loaded and contoured card skims even better, goes further, and faster, with some mass aboard to provide compression and momentum. Well, you can do the same with your surfmat. Gently lift the nose to gather pressure, both hydro and aero, and ride on a pressure cushion of increased lift. Press the outer pontoons down to make a concave cross-section, and your weight will more easily be carried across flat water. This skimming ground effect phenomenon is the key to shifting into higher gears.
I reckon that first gear would be kicking and paddling around. Second gear is simply riding the wave in a normal fashion. Third gear seems to result from extra pressure building up under the mat to provide a mysterious form of jet propulsion. One clue is that sometimes air accumulates under the mat in the grooves between the pontoons. This effect promotes super-acceleration, due to less fabric touching the water, and from high pressure air shooting out the tail ends of the channels. The feeling is most definitely like shifting from second to third gear in a fast car or on a quick motorbike.
On rare occasions I have experienced the good fortune of having strong offshore winds combine with perfect long walls to lift the whole mat clear of the surface to levitate like a hovercraft. Could this be fourth gear flying? I certainly assume so. At that point, with no drag on the water, and reluctance to trail my flippers on the wave face, the optimal method of steering seemed to be using my flippers as aerodynamic rudders, sort of like pushing the pedals in an airplane cockpit. The rush of wind and free floating sensation of skimming weightless was a major thrill!
As for fifth gear, or overdrive, well I'm still trying to get my head around what that might be...