I was reading about Automotive Brakes (Chapter 52, Automotive Mechanics by William H Crouse, Donald L Anglin, 10th edition). And in a section about drum-brake self-adjusters, it is said that the incremental self-adjusters are attached to the secondary shoe of a duo-servo brake; The adjustment is made when the vehicle is moving backwards and the brake is applied. And then the mechanism of how the adjusting lever and adjuster spring follows the text.
What I don't understand is why the secondary shoe? It only seems reasonable since the adjuster is doing its work while the vehicle is in backward motion. But then, why the adjustment only happens in backward motion?