I think most volvos are fwd biased and thats how the haldex system defaults. But when faced with front wheel spin power is shifted to the rear. An inherintly counterproductive arrangement if you ask me. As the vehicle starts rolling and unweights the front wheels, traction is lost and youre more likely to spin the wheels. with a rear-ward bias exactly the opposite occurs. Im not saying the rear bias is best for everything but for starting from a standstill it would seem that it is.