A true proportional controller (examples - BrakeSmart and Jordan) directly measures the braking effort the driver is applying to the truck (the BrakeSmart via hydraulic pressure in the brake system; the Jordan via brake pedal position) and generates a proportional braking output which is sent to the trailer brakes. If you push the pedal harder, you get more trailer brakes - every time.
An inertial controller has no direct connection to the truck's braking system. The only way it knows what's happening is (1. ) the brake light wire is telling it that the brakes are being applied and (2. ) its internal accelerometer is measuring the truck's rate of deceleration in an attempt to determine how hard the truck's brakes are being applied. In other words, the truck has to be decelerating before the accelerometer can determine how much trailer braking to provide. What happens on a slick surface when the truck cannot generate enough deceleration without locking a wheel to give the accelerometer the input it's seeking? One can find himself with locked truck brakes and no trailer brakes - instant jack knife (it happened to me on a previous truck and 5th wheel with a Tekonsha Sentinel inertial controller). For further illustration, see a previous poster's comments on using an inertial controller when hauling liquids - as the load shifts back and forth, the controller will sense the increasing and decreasing rates of deceleration and vary the trailer braking effort.
To overcome this critical deficiency, the Prodigy sends out a "boost" signal when it senses brake light activation, but that boost signal is purely artificial depending on what the operator has set in the controller. It has nothing to do with actual truck braking effort being applied.
Rusty