2003 GMC Sierra 1500 Pickup Owner's Manual - Page 297

Slower Speeds (below 40 m.p.h./ 64 km/h ) At slower speeds the front and rear wheelswill turn in opposite directions. This helps the vehicle make tighter turns, such as during parking, cornering and turning into tight spaces. Higher Speeds (40 m.p.h./64 km/h, and above) At higher speeds the front and rear wheels will turn in the same direction. This improves stability of the vehicle during lane changes and sweeping turns. (4 Wheel Steer tow mode): When towing a trailer the 4 wheel steer tow mode provides enhanced stability allowing the trailer to follow the path of the tow vehicle more closely, especially during lane changes. In this mode the system operates much like the 4 mode, but is enhanced for trailer towing. It is recommended for all types and weights of trailers. 4-16