lesr,

I have never had an issue with changing from "S" to "W" while moving, but I usually do that while stopped at a light or before I move after starting the car. When at a light though, the car is in "D" and not "P", so it's never been an issue.

That being said, I believe Alabaster04 is correct that it also alters the shift points of other gears to help keep the RPMs low and avoid spinning in inclement conditions.

I had thought I read somewhere that it affected reverse as well... although I could be mistaken.

Cheers!