I wouldn't be able to do a bend this quick with a lot of the highly figured koa where I'd be using a stainless steel 'strap' to support the outside of the bend.
The whole thing about having the iron vertical is you get a better picture of how the bend is progressing. You also retain a mental picture of the pattern better if it is in the orientation which you are working. My first guitar had it's sides bent on a gas pipe set horizontally and it was hard.
The pipe was vertical, but as I know now it was too small in diameter and too hot (I used a one inch pipe and a heatgun). The mahogany I had for the sides was also a bit too thick (2mm) I think. I will definitely try to get thinner wood next time.