Electric trailer drum brakes:
Two brake shoes, primary (front) & secondary (rear)
Actuating arm
Cam - pivot
adjusting arm-link-star wheel assembly
magnet
smooth finished interior face drum (hub side)
The magnet is loosely attached to an actuator arm at the bottom of the brake assembly
When you apply current to the magnet, the magnet attempts to attach it self to the drum. If you were looking straight at your wheel from the side, the magnet is moving towards you, 90 degrees in relation to trailer movement.
As current is applied, the magnet moves to the drum face
The rotation of the drum, with the magnet trying to attach itself causes the actuator arm to move.
At the top of the now moving actuator arm there is a cam assembly between the tops of the two brake shoes
Arm is moved by the magnet, cam rotates, the brake shoes start to spread at the top of the drum.
As the shoes start to apply, the front shoe now attempts to rotate with the drum
This movement, through the star adjuster assembly now pushes against the bottom of the rear brake shoe, forcing it even harder into the drum then just the cam action alone.
That is why the front shoe is smaller then the rear shoe, the front shoe is more of an actuator and the rear shoe is the worker.
As current varies to the magnet from your brake controller the "pull" of the magnet varies. The harder the pull, the more the magnet is pulling the actuator arm. The more teh arm moves the more the shoes are applied, the harder the front shoe is pushed into the drum the more it pushes the rear shoe.
Some trailer manufacturers will bias the brake application, e.g. 60% front axle 40% rear axle for the same reason as your vehicle, the front brakes do more of the stopping. This is also a good reason to have your trailer as level as possible when towing. Even if the axles where not biased, a nose high trailer will have slightly less braking with the front axle as compared to the rear axle.
This is also why the average brake controllers work the way they do. Some are a time delay, some are proportional (usually in relation to vehicle movement).
The simple answer with the better proportional controllers is, as voltage is applied and the controller sees a decrease in speed, more current is applied to the trailer brakes. Some controllers use a pendulum to achieve this, which is why the installation instructions will list a maximum out of level mounting position.
There are true proportional controllers available, but few use them, and they can be pricey. Typical is a pressure transducer in the trucks hydraulic brake system. More pressure applied with your foot, more current to the brakes.
The 2011 and above Super Duty's have a true hydraulic activated trailer brake controller, and it works really good.
Wider, larger diameter brakes shoes will apply more force-friction then smaller shoes with a given amount of current, assuming the magnets were the same size.