Some explain to me how Bone plating works.

Zed level 4 did half damage to me without it. Zed level 4 another game did 10% damage estimated with bone plating. How does it work.

I dont know when its triggered. At the bottom of your screen, theres the icon and its a timer but grey. is it triggered and ready?

Also, is it only good against burst champs? If so, what kind and type?