Can anyone explain how Gargoyle Stoneplate works?

Active wasn't working on PBE on release. It is now. Look the newest Brofresco video

how it should work should be simple--

it increases your armor/MR if you are by 3+ enemy champs, but you can also click activate it to increase your health dramatically while reducing your damage, the increase is increased even more if you are by 3+ enemy champs.

It sounds like a team fight item for low MMR people that don't know how to focus on the adcs or something. I could see it being very well on champs like Renekton who are tanking and also have stuns, or Illoai who is tanking and still does good damage