The bar connects the two shock absorber mounts. In a hard corner, the loaded side of the suspension pushes outward and the geometry deteriorates precisely when you need it most: the bar distributes the load between both sides.

The front end becomes more precise when turning in, and the car maintains the same response from one corner to the next. The effect is clear on older chassis, convertibles, and high-mileage cars; it is subtler on a modern, already rigid body.