This is what I found "Differentials- Locking Type (Speed-Sensitive) Tractech’s locking differentials are the NoSPIN® and Detroit Locker® brands (same product, different market segments), the Detroit E-Z Locker™ and Detroit Gearless Locker™ brands. They keep the wheels locked together (except when turning) so that together the left and right wheels, always deliver maximum traction to the ground; neither wheel can spin out.
They allow different wheel speeds in a turn by disconnecting the faster - moving wheel (usually the outside wheel - driven faster throughout the turn), driving the vehicle with the other (inside) wheel.
http://www.tractech.com/Products.htm