Side B: Reverse Flowing Time
On Side B, it’s Side A that looks like it’s happening in reverse, but in actuality it’s you that’s inverted. Which brings up the reason those oxygen masks are needed, as once you’ve inverted, your entire body exists as an inverted machine. As such, the inverted membrane of your lungs cannot handle standard air, and you’ll need to reverted to breath normally.
Tenet sums this up rather neatly in a piece of dialogue: “You’re inverted, the world isn’t.” So when The Protagonist saves Kat in inverted time, this creates the necessity to bring her to the Oslo Freeport to revert her back to normal. Not only does she not need the telling oxygen mask that would out her as another inverted version of herself, but she’d be able to speak normally. As such, the now reverted Kat 2.0 is now able to go to Vietnam, sans oxygen mask, and try to stop Sator from killing himself; while her original self goes on a trip with her son.
