In the mid-70s, Porsche developed this unique rear suspension for its award-winning 928. It is a variant of semi-trailing arm suspension.
In any suspensions, the pivot joints must be inserted with rubber bushing to absorb noise and vibration. For conventional semi-trailing arm suspension , whenever under braking, the momentum of the car body tries to pull the car "away" from the rear suspension. Due to the elasticity of the rubber bushing, the rear wheel will toe-out. As a result, the car will oversteer.
Weissach axle was designed to eliminate this oversteer By splitting one trailing link into two pieces, with a pivot joint added between them, the oversteer under braking can be elinimated or can even introduce some understeer ! This help in stablizing the cornering motion. Some people call it "passive rear-wheel steering".
The methods of introducing passive rear wheel steering differ but many makers are now going down this track.