Sway Bar, Anit Roll Bars, or Stabilizer Bars...
Different names (all the same thing)
Sway bars or Stabilizer bars control the vehicle's body roll or lean during a corner.
What that means is that sway bars can have a large effect on "weight transfer" and a tire's "contact patch". Hence, oversteer or understeer can be produced/controlled by adjusting/changing the front/rear sway bars.
Through the just the use of sway bars alone, we have seen increases in grip from .03 - .10 g's...pretty significant!
Rear Sway BarOne of the ways that BMW/MINI (and most other auto manufacturers) make sure some bone head isn't going to sue them is they dial understeer into the chassis handling dynamics.
A more balanced car (or one with a nice little bit of slight oversteer) can be had by installing a larger, thicker (read stiffer) rear sway bar.
By changing out the stock rear sway bar with a thicker (19 mm or 22 for even more aggressive handling), you increase the rear end's resistance to roll. If possible, get an adjustable rear sway bar?it will allow you to adjust under/oversteer to your driving conditions.
Increasing the rear end?s resistance to roll helps to transfer weight to the front outside tire during a corner. More down force on a tire means more grip.
Hence, transferring more weight to the outside front tire increases the front end's grip.
That is why everyone says to tighten up the rear end to reduce understeer.