Winter mode works great in slippery conditions such as rain, snow, or ice. What this mode does, is basically start your car off in 3rd gear rather than first gear. The RPM's will go up much higher than usual to start moving the car. You should always leave your car in normal mode, and only use winter mode when necessary. If you come to a stop and feel your rear wheels might slip, just press the "W" button and you should be fine. If your car has traction control also, you should generally be alright for the winter.
Winter mode is not going to get you out of the worse conditions, but it will help you out. The 960/S90 also has an automatic rear center locking differential, which also helps to get out of slippery conditions. If one wheel starts to spin, it will automatically lock both rear wheels together. The best thing you can do for winter is to have good tires with lots of traction.
-Drew
