COY stands for Cross Over Yuri. The premise is pretty simple, I have a list I've compiled of over 600 cute girls from various anime and manga. I then use a random number generator to make pairing, which I then draw. Every third picture is a threesome.
I had a bit of a dilemma with this one, in that I very nearly had Midori still on Seiji's arm for this picture. In the end I decided that the image itself would likely suffer from this, as I couldn't find a way to make this sexy to me. It's one thing to have a tiny girl rubbing against a giant cock, it's another entirely to have her suffocating inside another girl's vagina. So, I opted to have her be in her true form, even though she has very few appearances like this in the anime.
Of course Ropponmatsu had to be on top. :P