It should come as no surprise that surfboards can be considered small, hulled vessels- like little boats or "hulls". From a hydrodynamics standpoint, nearly all popular surfboards should be sub-categorized as "displacement" hulls because of the ubiquitous presence of tail rocker. A board's rocker, or bottom curvature (when viewed from the side), is responsible for wave drag; the more curve, the more drag. While any hull will plane if enough power is provided, displacement hulls will never be as efficient as planing hulls because of this drag. Planing hulls eliminate drag via a rocker-less (flat) tail section stretching from the wide point of the board to the tip of the tail. Ever wonder why retro fish feel so fast? It is because they have very little rocker compared to a performance shortboard. The mini-simmons is a further extension of this concept, where the goal is to eliminate tail rocker entirely. A rocker-less hull will plane ultra-efficiently and enable the rider to approach waves in a unique way. Shortboards will mire and sink unless they are constantly positioned in the power center of the wave. By contrast, planing hulls will continue to plane even when they are, by comparison, far from the wave's power center. Sure, professional surfers will never compete on a planing hull- drag is a necessity if you're trying to get barreled or do gouging turns, but for someone seeking a completely unique surfing experience, a planing hull is like entering another dimension.
Bob Simmons is credited as the person who first applied the planing hull to a modern surfboard. He mostly experimented with long boards but, over the years, his ideas have morphed into boards of all shapes and sizes. Differ though they might, they all share one common characteristic- very low tail rocker, creating a higher efficiency board.