On the left side, lowering...
On the left side, lowering the bottom bar raises traction but can cause the car to push coming off the corner. Raising the bar allows more rear steer, which can often make the car feel more comfortable to the driver, but doesn't necessarily aid traction.
Bar Length
Because of the different angles involved, the upper bar should always be longer than the lower bar on the same side. Wilson says that the bar length should always be adjusted so that the attachment points on the birdcage are at 6 and 12 o'clock. Your baseline should be with the lower bars on each side the same length as well as the upper bars on each side. If your race car is tight all the way through the turns, you can lengthen both the upper and lower right-side bars. Shortening the right-side bars will tighten the car all the way through the corners.
Conclusion
That's a lot of adjustment options, and we only discussed rear suspension bar length and location. Wilson says one easy way to remember what does what is to limit your adjustments according to where on the track your car is having trouble. Right-side chassis changes affect the car most off-power, so if you are having handling problems on turn entry, make adjustments to the right-side suspension arms. The left side has greater influence on the car under power, so go there if the problem is on turn exit.