I'm not sure if it is the shape and multiple elements of this fly that make it effective or the simple fact that it really gets deep fast to where the fish are eating. I suspect it is the latter, with more of your drift being in the column's hot spot.

Either way it does work well. Size effectiveness seems to depend on the size of the naturals. Color hasn't seemed to make much of a difference for me, but I tend to use them mostly in red and blue.