Can't speak as to how a Brake Buddy works, but my Ready Brake uses a cable to pull the brake pedal toward the floor. And, yes, it is a strong enough pull to engage the brakes even when there is no power to them.
Jeff, Tricia, 3 kids, and the occasional dog or two
2006 Winnebago Tour 40KD, 2013 Dodge Ram TOAD
350 Cat C7, Freightliner chassis