Okay, I'll probably use some incorrect terms here, but...

The gentle folds on either side of the back of the trachten seem to follow similar lines to some sewn-in "darts" (?) on either side of the back of my PC. The small "belt" on the back of the trachten seems to tailor the back, giving it a more "fitted" effect without sewing that effect into the garment the way it appears to be done on a PC. To me, it's a nice, relaxed look, without being a formless sack.

I'm wondering if there are examples of this kind of approach in Highland attire (of any period).