Side B: Reverse Flowing Time
On Side B, it is Side A that appears prefer it’s occurring in reverse, however actually it is you that is inverted. Which brings up the rationale these oxygen masks are wanted, as as soon as you’ve got inverted, your whole physique exists as an inverted machine. As such, the inverted membrane of your lungs can’t deal with customary air, and you may must reverted to breath usually.
Tenet sums this up quite neatly in a chunk of dialogue: “You’re inverted, the world isn’t.” So when The Protagonist saves Kat in inverted time, this creates the need to carry her to the Oslo Freeport to revert her again to regular. Not solely does she not want the telling oxygen masks that will out her as one other inverted model of herself, however she’d be capable to converse usually. As such, the now reverted Kat 2.zero is now capable of go to Vietnam, sans oxygen masks, and attempt to cease Sator from killing himself; whereas her authentic self goes on a visit along with her son.