The first time Kowalski saved Stone’s life was earlier, when he told her to detach. The second time is in another post impact scene that mirrors the first. Only this time, the roles are reversed. He’s the one about to drift into infinite blackness. We see that he’s dragging her with him and the only chance for any of them to survive is if he cuts off the rope. In other words, Kowalski saves her life again through detachment. It’s not by preventing her to float with him into space, but by teaching her that sometimes it’s ok to let go, both literally and metaphorically. “You have to learn to let go,” he says. It’s a beautiful scene that should feel therapeutical for anyone carrying past grief on their shoulders for far too long.