Per Samsung power save mode will: "Save battery power by limiting the maximum CPU performance, reducing screen brightness and frame rate, turning off the touch key light, turning off Vibration feedback, and reducing the time before the screen is turned off when notifications are received."
Apparently that last bit is imprecise, as the screen turns off quicker even when you turn it on, not just when notifications are received. Samsung's own measurements only show about an extra 30-40 minutes of usage in Power Save mode versus regular mode. This comes out to be 11 hrs 50 minutes versus 11 hrs 20 minutes. Not a big savings IMO.