...on horizontal surfaces, [the vertical polisher] is much easier to control and use rhythmically in a back-and-forth motion than the "flat" style. It's easier to keep it at the required angle, and (used by right-handers) it gives your left hand more control when it pulls, because your left hand is pulled in tighter toward your body (sorry if this explanation is hard to grasp).
On vertical surfaces, however, the standard (flat?)configuration is easier to use; the vertical polisher is harder to hold and more tiring for your right hand. Also, your right forearm ends up taking the brunt of the weight, and you get tired more quickly. With the other style, you get a more natural "sweep" and leverage, as you don't need to keep your arms held as tight to your body, and your shoulder muscles and left bicep hold up more of the weight, which makes it easier to control.
I'm sure my description is hard to imagine...