I was just watching one of the many youtube recap/breakdown vids that mentioned it *right now.*
Awesome eerie coincidence.
It is an injection of hyperoxygenated blood. If she has any oxygen in her lungs, it would have expanded immediately upon flying out of the ship, killing her. That’s why Cyn dies & she doesn’t.
So once she is floating & has pushed herself as far as possible without breathing, the injection gives that oxygen. But she wants to hold off as long as possible because she could die IF it takes too long.