Brake bias seems like a good guess (per Big Mangalhit & Sieper). In conjunction with the foot pedal, the system could operate like a fore-aft version of the banned fiddle-brake. When modulating such a lever, would you want the car to have a tendency to straighten out (front bias for understeer), or rotate (rear bias, oversteeer)? Intuitively I'd say set it to shift bias rearward so that turn in is enhanced when needed. Additionally, would there be any advantage to engaging extreme bias ratios with such a lever? As in, 100% forward or rearward.
If it's harvesting bias (per MuseF1), Vettel would have finer control over MGUK activity.
Third option: wastegate actuator? Engagement of electrical-supercharger/full-power/quali mode as needed.
Relevant regs:
11.1 Brake circuits and pressure distribution :
11.1.1 With the exception of a power unit, all cars must be equipped with only one brake system. This
system must comprise solely of two separate hydraulic circuits operated by one pedal, one
circuit operating on the two front wheels and the other on the two rear wheels. This system
must be designed so that if a failure occurs in one circuit the pedal will still operate the brakes
in the other.
The diameters of the master cylinders acting on the two rear wheels and the two front wheels
must be within 2mm of each other and have the same available travel. The same principle
must be applied in multi-stage master cylinder designs.
11.1.2 The brake system must be designed in order that the force exerted on the brake pads within
each circuit are the same at all times.
11.1.3 Any powered device, other than the system referred to in Article 11.9, which is capable of
altering the configuration or affecting the performance of any part of the brake system is
forbidden.
11.1.4 Any change to, or modulation of, the brake system, other than any movement of the minimal
flexible parts described in Article 11.4 to 11.6 [Brake ducts.], whilst the car is on the track must be made by the driver's direct physical input or by the system referred to in Article 11.9, and may not be pre-set.
11.7 Brake pressure modulation :
11.7.2 No braking system may be designed to increase the pressure in the brake calipers above that
achieved by the driver applying pressure to the pedal under all conditions.
11.9 Rear brake control system :
The pressure in the rear braking circuit may be provided by a powered control system provided
that :
a) The driver brake pedal is connected to a hydraulic master cylinder that generates a
pressure source that can be applied to the rear braking circuit if the powered system is
disabled.
b) The powered system is controlled by the control electronics described in Article 8.2. [a.k.a the ECU and associated wiring and sensors.]