“Wow, this looks soo fluffy!”
You could use an inverted light vector to lighten up the back faces but that would be too easy. :D
They modified the normals. This means they used the inner “bubble” object as a base and “projected” the normal orientation of it to the leaf “cloud”. So you don’t get too dark/bright faces in the wrong place but a very nice and soft shadow gradient all over the leafs.
If you think i didn’t explain it good enough, feel free to read more about this in the polycount wiki. There you also find the mentioned script which makes the copy process possible. And of course this link, which is so awesome i have to post it here: It’s about the shading of gras.
Warby told me another very nice advantage about this technique: You avoid too many transparent planes being rendered over each other. The big blob mesh in the middle culls most of them so you only need to care about the stuff of the front/sides.