[halfmoonclip]

I've been following some discussion on the matter here, but a line in the 'BMW X1 Quick Reference Guide' caught my attention.
Page 29 says specifically:
"The system identifies a loss of pressure by comparing the speeds of rotation of the individual wheels as the car is traveling."

Other references seem to indicate that there is some electronic component in the wheel.
So which is it; electronic sensors or tire rotation?
When buying replacement wheels (for winter tires, for instance) is it necessary to buy wheels with sensors?
A really handy feature of our A4 Audi is the ability to install the winters here at home, with no need for the dealer to reset for the replacements; tire rotation alone keeps you informed if there is a tire down.

Help me out here, and thanks,
Moon