The Big Sky Hi Aspect Twin is smaller and more refined than most modern day twins.
Initially, Andrew Kidman worked with a slightly larger version of this fin with Larry Gephart. Over the years, they continued to make the fin smaller, seeing what they could get away with before the fin became too small. Eventually Andrew settled on this size.
The smaller the fin, the less wetted surface area, the faster a surfer can go. The double foil helps control this speed and the cavitation caused by turning at high speeds.

Why the Double Foil? 
The Twin Keel and High Aspect Twin both feature a double foil.
Basically, this allows the water to wrap around the fin as the fin travels through the water, similarly to the centre fin on a Thruster or Single Fin. The double foil helps stop cavitation in turning, as the water sucks around the fin rather than breaking off the edge of the flat inside surface. This gives the surfer more control and stops the fin from breaking free in the turn. It's a smoother feeling.

BASE: 140mm / 5.5 ”
DEPTH: 135mm / 5.3 ”
FOIL: 50/50 Centre Foil
SYSTEM: Futures compatible