Narrower tire creates more pressure on the contact patch (which will be smaller) and it will get somewhat better grip in the snow/slush/ice. Dry grip will suffer, so it's a bit of a balance game (in other words, how much does it snow where you live?).

Have you ever run on ice, and found that if you run on your toes you get grip, but if you flat foot you don't? Same concept.

Not sure if the knowledge is "common" outside of the areas which get a lot of snow. Tire Rack's recommended sizes tend to go a couple sizes narrower than OEM, so they seem to have figured it out.

 

Check out the skinny tires they use in the rally of Sweden.
http://www.7extrememotorsports.com/w..._206wrc_12.jpg

 

The worst decision I ever made was to be lazy/cheap and leave my ES100's on my RSXS for the winter.....it was quite amusing. I did get Yoko Avid V4s for this winter so I know it will be at least a lil bit better. The stock michilins weren't great either in the winter weather.