How does ConfigurableJoint's TargetRotation works?

When the script reference doesn't yield answers, I check the reference and user manuals. Here's the much more thorough docs on how to use a ConfigurableJoint.

After a quick read of the section on rotation, I would ask, "did you properly setup the rotational acceleration?" This would be either the two angularDrives or the slerpDrive as defined by the rotationDriveMode.

Movement/Rotation Acceleration

 <p>You specify object movement or rotation in terms of moving the object toward a particular position/rotation, or velocity/angular velocity. This system works by defining the "Target" value you want to move toward, and using a "Drive" to provide acceleration which will move the object toward that target. Each "Drive" has a Mode, which you use to define which "Target" the object is moving toward.</p>
 
 <p>...</p>
 
 <p><strong>Rotation Acceleration</strong></p>
 
 <p>Rotation acceleration properties: Angular XDrive, Angular YZDrive, and Slerp Drive function the same way as the translation Drives. There is one substantial difference. Slerp Drive behaves differently from the Angular Drive functionality. Therefore you can choose to use either both Angular Drives or Slerp Drive by choosing one from the Rotation Drive Mode. You cannot use both at once. </p>