Gene01

Yaw is rotation about the vertical axis (in this case, going through the center of the detector).
The yaw rate sensor only measures the RATE of heading change (i.e.the angular acceleration), not the deviation from its original heading.
You could be heading 90 deg. from your original heading, but if you stop turning, the yaw rate sensor will indicate zero.

rkw77080

Precisely the reason to do the zero-calibration. You don't want the ECU to "think" that you're experiencing angular acceleration when you're not (and take the wrong action). Again, most day-to-day driving will have no need for this feature - only when you're skidding out-of-control (large deviation between yaw rate and steering angle) that the VSC needs to give you a hand.