The Weissach axle (pronounced 'Vise-ock') is a special rear suspension arrangement devised for the Porsche 928. The goal of the Weissach axle was to eliminate lift-throttle oversteer by allowing the rear suspension to adjust itself during cornering maneuvers.
The Weissach axle is a variant of the semi-trailing arm suspension. The tendency of a vehicle to oversteer when decelerating is compounded by the compliant bushings found in most trailing arm suspensions. When the vehicle is decelerating, the trailing arm pivots towards the rear as the wheel is "pulled" backwards relative to the chassis. This results in toe out, which makes the vehicle unstable.
For the Weissach axle, the front pivot bushing of the trailing arm is replaced by a short link. In this arrangement, when the vehicle decelerates and the wheel is "pulled" back, the result is toe in. This adds to stability and thus, reduces oversteer.
A similar, but simpler concept to the Weissach axle is seen in the rear suspension of the second generation Mazda RX-7. Instead of an extra link, a special bushing is used in the same position in the lower control arm to perform a similar function.
Suspensions with this or a similar function are sometimes called "passive rear steer". The concept of passive rear steering has fallen out of favor for new car designs, as semi-trailing arm suspensions have almost completely given way to multi-link designs with dedicated toe control arms.