The whole world is flipped. We don't know if it will change anything else yet. It's not a mode I really use myself, but in a game you'd play through multiple times, it's a pretty neat way to change things up a bit. It was also in the Shadow of the Colossus remake and I know a lot of the vets appreciated the change up.
Another thing to note is that, if it works like in other games, it'll make your left hand the main hand. Meaning you have to flip how you equip weapons, shields & parry weapons.
Thanks for the details.
On paper it looks pointless and idiotic to me... Might be better playing it. I'll see xD