From greencarcongress:

The BMW iFE.18, with which the BMW i Andretti Motorsport team won the opening round of this season’s ABB FIA Formula E Championship, consists of many features developed internally at BMW, as well as the standard parts stipulated in the regulations (chassis (Spark Racing Technology); battery (McLaren); tires (Michelin)). The most obvious of these is the Racing eDrive01 drivetrain. (Earlier post.) However, the engineers at BMW i Motorsport had other areas in which to apply their know-how. These include:

Brake-by-wire system. Another major in-house BMW development is the electronic, brake-by-wire braking system. The introduction of this system represents a big technological step for Formula E and, in this regard, takes it to the same level as Formula 1 and the LMP1 category in the FIA World Endurance Championship.

As of Season 5, brake-by-wire controls the relationship between mechanical braking force, when the driver pushes the brake pedal, and the braking effect generated by energy harvested during deceleration and braking.

When the motor recovers energy from the braking process during the race, it basically functions as an additional brake on the rear axle of the car. Previously, drivers had to manually adjust the brake balance to balance the additional braking effect. The electronics on the BMW iFE.18 now perform this balance.

Read the full story at greencarcongress.com.

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In addition, here is how BMW described its brake-by-wire i Motorsport tech in a press release:

Brake-by-Wire system.

Another major in-house BMW development is the electronic, brake-by-wire braking system. The introduction of this system represents a big technological step for Formula E and, in this regard, takes it to the same level as Formula 1 and the LMP1 category in the FIA World Endurance Championship. As of Season 5, brake-by-wire controls the relationship between mechanical braking force, when the driver pushes the brake pedal, and the braking effect generated by energy harvested during deceleration and braking. When the motor recovers energy from the braking process during the race, it basically functions as an additional brake on the rear axle of the car. Previously, drivers had to manually adjust the brake balance to balance the additional braking effect. The electronics on the BMW iFE.18 now perform this balance.
 
The integration of the brake-by-wire system and, above all, the software logic developed by the BMW i Motorsport engineers, significantly increases the regeneration potential – the maximum amount of energy that can be fed back into the battery when braking. Finding the best possible configuration of this software is crucial to be able to contest the entire race distance with maximum power from the battery, whilst complying with the amount of energy permitted by the regulations. In practice, overcoming this challenge requires the engineers to achieve the best possible relationship between speed and energy management. Their task is to calculate a race time optimisation with the limited amount of energy. It is important to keep sight of all the race action – to read the data and to anticipate what may be coming – to then decide, together with the driver, in which phases of the race it is better to save energy and when to attack, to make up positions or open a gap to the cars behind.