Active Booster
WARNING: This page is about a different car, the 2005 Nissan Titan. However, it is still accessible from the selected car via links, so may be relevant.
The active brake booster consists of vacuum booster, an active booster control group and a delta stroke sensor. In case of brake booster system malfunction due to loss of vacuum the delta stroke sensor will signal the ABS actuator and electric unit (control unit) that a booster failure has occurred. The active booster then applies supplemental force to the master cylinder relative to the amount of force exerted on the brake pedal.