Press it when you are stopped and in gear.
The winter light will come on on your dashboard just above the fuel gauge.
The car will shift into 3rd gear and will drive in that gear until you hit 15mph. After that winter mode will be disabled and the car will operate normally.
As long as you stay below 15 mph, the car will attempt to reduce wheel spin by keeping the torque at the tires low by driving around in 3rd gear.
That's all it does.
I have never used it, but it does what it says. I just respect the road conditions when they get icy and keep a very light foot on the gas and my speed low.
I migth be wrong about 15... For some reason, now I think it is actually 18mph?
It works in 2WD and 4WD IIRC.