My personal hero Kai Troester said it's mostly sodium, and it might keep your mash pH from rising over 5.8, at best, but probably doesn't do anything. It definitely adds about 100ppm of sodium to your beer. So, if you like salty beer, 5.2 stabilizer is really cool. In fact, it's the best!
It's just not physically possible to make a phosphate buffer that buffers to 5.2, end of story.