A stiffer rear bar puts more loading on the outside rear tire, so if you lift in a turn, it will transfer the weight to the front, and that loaded outside rear tire will step out slightly. It's not a dramatic loss of adhesion with tons of tire squealing, the rear just briefly describes a slightly wider arc than the front (how is that for some technical jargon?
). But it helps to point the car into the turn.
BTW, this winter I replaced both front and rear sway bars with ROW M030 bars, and I've noticed that the car doesn't rotate as well as before; it feels less "lively". I've got 20mm wheel spacers at all four corners, so I may go with 15mm spacers in the rear to see if that makes any difference. If not, I'll switch back to the stock spec front bar. The rear bar is incredibly easy to change, unfortunately the front is a bit of a project to replace.