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Toe-In
When a brake shoe presses against a moving rim, the pull of the rim causes the brake arm to flex a bit. If your brake shoes hit a stationary rim perfectly straight and squarely, the flex of the brake arm will cause the rear edge of the brake shoe to do the brunt of the work. The front edge of the shoe may not even be engaging the rim under hard braking.
Good practice in installing brake shoes is to "toe them in", so that the front part of the shoe hits the rim first. As the brake arm flexes under real braking, it will permit the whole surface of the brake shoe to engage the rim.
Toeing in of brake shoes can also reduce the annoying squeal some brakes make when in use.
Brake shoes that are longer to the rear of the post will press more evenly on the rim. Most newer brake shoes have special washers with curved surfaces to allow you to adjust the angle of the shoe to the rim. Older brakes relied on brute force, typically bending the brake arm with an adjustable wrench. That is not a good idea with aluminum brakes-- it can initiate cracking, which leads to failure.
