It refers to the shape of the freebag in which the reserve canopy is stowed. Think of a molar tooth. The freebag is sewn with webbing in the middle from the grommet out to the edge of the bag, the webbing continues on as the bridle. This forms two pockets on each side of this webbing where the "ears" of the canopy are stowed. When you look at a freebag with a canopy in it, it resembles a molar tooth.