Asymmetrical designs are meant to create a balanced blade by creating equal surface areas on each half of the blade as it enters the water. The right amount of asymmetry will reduce fatigue by eliminating torque, or twisting of the paddle, when the blade enters the water. Typically the lower part of the blade hits the water first and will torque or twist if there is not asymmetry. The right amount of asymmetry creates a smooth entry of the blade into the water allowing you to use a light grip to reduce fatiguing your hands and arms.
Asymmetry maximizes the power of the blade as it is inserted and takes into consideration the angle of attack. However, the perpendicular asymmetry must be balanced by a longitudinal symmetry or the blade will want to twist in the water. This means that there must be an equal surface area on either side of the centerline of the blade regardless of the depth of insertion.