What is ASR and what does it do?

It stands for Anti Slip Regulation AKA traction control. let me quote the service manual. this is such a KILLER feature.
Traction control will not have any affect on the operation of the vehicle until the control module detects one or both of the rear wheels rotating faster then the front wheels. At this time the electronic brake and traction control module (EBTCM) will request the power train control module to retard the timing. If the EBTCM continues to detect the rear wheels are rotating faster then the front it will request the ASM to reduce the throttle angle. The last measure taken to control the rear wheel is for the EBTCM to apply the rear brakes, thus reducing torque to the rear wheels. Once the rear wheels being to rotate at the same speed as the front wheels, the system will return full control to the drivers. During traction control mode, if the brake is applied to only on rear wheel, most of the torque from the engine will be directed to the other rear wheel which will improve the traction of the vehicle.

The braking is accomplished by closing the TCS master cylinder isolating valve, this isolates the master cylinder from the rest of the system. The TCS prime valve opens to allow the pup to get brake fluid to build pressure for braking. The drive wheel circuit solenoids are energized as needed to allow for pressure hold, pressure increase or pressure decrease.

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