Russ, ZF makes both an active and a passive steerable tag axle. Newell uses the active steerable tag. While both of these tag axles function much like a non-independent front suspension, the passive tag uses steering dampers and pneumatically actuated lock cylinders to allow the tag wheels to turn together but limit the amount of turn. The active steerable tag uses a hydraulic cylinder to control the steering. The cylinder keeps the tag parallel with the drive axle most of the time. When the front wheels turn past 5 degrees, the hydraulic cylinder begins to turn the tag axle in the opposite direction. As the front wheels continue to turn sharper, the tag axle increases up to a maximum of 23 degrees. According to Newell, the turning radius of a 45' Newell is reduced from 44' without a steerable tag to 37' with the steerable tag.
See
http://www.zf.com/media/media/en/doc...systeme_ZF.pdf for drawings (including the one above).