"Kate Flashes Jack" is an orchestral piece from the Season 6 soundtrack. It plays during the series finale.
And for a moment, he does remember her. He remembers their formally introducing themselves by the campfire the night of the crash. He remembers their first meeting, when she stumbled upon him, examining his wounds. Kate tells him she has missed him, and he remembers more - he remembers her kissing him in the jungle. But he turns from her and remembers no more, leaving her disappointed. He instead demands answers, and Kate says he'll understand what's happening - if he comes with her.
Back on the Island, Jack, Ben, and Hurley arrive at the cave of the light.
This piece uses the Light theme.
In this scene, Kate triggers Jack's flash.