I have the same question. Not sure if this is it but in looking at the photos the horizontal row keeps the weight perpendicular to the body and whereas the one arm row keeps it parallel to the body with the elbow tucked in more.
Hi SunKissed. If you click here and scroll to the bottom, you will see a muscle animation tutorial which addresses all aspects of the one arm row. We will feature more of these shortly so be sure to check out the workout manager frequently for these tutorials as well as Cathe TV on the bottom Right.