I've always enjoyed paddling backwards as well. I think the feeling of it being easier (hydrodynamically speaking) is due to the paddling position and bracing. I generally paddle backwards strictly by torso rotation, just plant the paddle and twist. This uses the big muscles of the back and since you're bracing directly off the back band or seat back, it seems like it takes less effort. I don't think it does, though - check the glide after getting up to speed forward vs. backward, that would tell the tale.
Paddler's Truck Rack