Seattle Zam project here, Deunan Shiruba. I have a theory for those two mysterious "butt pillows" that you can just see in the spinning Zam picture, under the skirt.
Have you noticed that whenever we make a Zam outfit, the vest works itself free in the back and pops out?
But in the movie, the actress runs for minutes and minutes, continuous shot, and it doesn't!
I think those butt pillows are possibly WEIGHTS. I think they are extensions on the bottom of the back of the vest to keep it down. They double as sitting cushions (we know how comfy that leather skirt is, don't we?).
It's a theory.
Since I've already completed my vest, I'm using a "belting" or a "belt stiffener" in the sides to hold it straight. I might experiment with velcro in the back to hold the vest in place, but it might rip loose when under tension from bending over or sitting. Experiments continue.