During this manoeuvre, if things are going well for the driver, then the car is held with all four wheels right at the very limit of skidding but without the car deviating from the driving line that the driver wants to follow through the corner.
If things are going slightly less well, then the front wheels might start to slide a little more than the rear wheels, giving the driver a lazy, understeering car that will not turn. If things are going less well still, then it might be the rear wheels that slide more than the front wheels, and if they slide too much, then the car will start to spin.
For this reason, the driver cares enormously how much braking happens on the front wheels compared to the rear wheels. If the car is unstable, and wanting to spin on corner entry, then you probably need to ask less of the rear brakes, and more of the front. If the car is lazy and understeery, then you would do the opposite.
Furthermore, as the corner progresses from initial braking to turn-in to corner apex, the driver wants different things. As the car starts to turn in, the car can often have a natural tendency to oversteer which is progressively replaced by understeer as the apex approaches.
This tendency can be counteracted to some extent by a clever braking system which would ask less of the rear brakes on turn in (to stabilize the car) and then ask progressively more of the rear brakes (compared to the front) as the apex approaches. This clever process is called Brake Migration - a dynamic change of the brake balance as a function of the brake pressure.
It is this cleverness that the Brake By Wire system provides. Guided by the rotary switch settings that the driver has made on their steering wheel, the BBW system juggles the braking input of the three main actuators (the callipers, the engine and the MGU-K) to provide the driver with a smooth, predictable shape to the rear braking action that allows him to keep the car at limit of adhesion (without any anti-lock brakes) while steering the car through the braking phase of the corner.