The Erect Directional are a alternative fin set up. They are fast smooth and predictable. They do not grab as tightly on that rail, allowing the surfboard to move faster through the water and recover from tight turns quicker without burying the rail.  With a shorter distance from the leading edge to the trailing edge of the fin, the water is released quicker. With a regular fin there is a much greater distance from the leading edge to the tip of the fin meaning that water tension on the fin is held longer, making it slower. 
Height   6"
Base     3.5"