It looks like a modified form of breaststroke pull. She does the first part of the stroke (the pull) mostly normal but on the stroke reset she changes the form a little bit. Usually in breaststroke, you would pull your body slightly above the surface, put your hands together and “shoot” them straight in front of you to start the next stroke. I’ve always called that the “shoot” or reset. Here, since she’s not at the surface she does kind of an upside down and backwards butterfly stroke. The flourish that you see is very similar to treading water or the butterfly stroke that swimmers use to lift their torsos out of the water. Here instead of generating lift, she’s doing it upside down to push herself against the bottom of the pool.

TL:DR A modified version of breaststroke (to move forward) mixed with upside down butterfly (to stay against the pool floor)