Yup, thanks for pointing this old trick out.
Since mobiles do not care much about polycount, but more about pixelshaders. I think this duplicate geometry trick will do well on mobiles.
I am not at my home desktop now, but what i remember from my materials:
I fake global directional light, because its much cheaper, i can get some “phong” fake lighting model this way, all with dark surfaces that are in shadow (no shadows casted).
You would want this to make darker (shaded) are on models. It is simply dot product of normal map, world normal, and lightvector (that i supply as parameter).
Currently i am trying to make (fake) shiny metallic reflections of global light, but android for some reason cannot produce bright pixels at all. Everything at best is what you would get with 0.75 brightness on PC in editor, and that is when i supply 4 up to 16 as brightness value in android. I know it cuts that to 1 if there is no postprocess with bloom. But finding reasons for this is for later time when i do facelifting pass on my game.
Ps. well then ios is opposite to android in regrads to polycount vs shaders