the idea of it improving battery is that generating frames is less performance intensive than running a certain framerate (e.g 60 fps capped game with frame gen at double the framerate consumes less power than running the same game at 120 fps). though its slightly less practical because frame generation only makes sense when the base framerate is high enough (ideally above 60) to avoid a lot of screen artifacting. So in practical use, this only makes sense to “save battery” in the context that you have a 120hz+ screen and choose to cap framerate to 60-75fps