My car is 4Matic and I've got Nokian studded tires, so it can accelerate pretty good on slippery, snow covered roads. Last evening we had a good snowfall and the roads were quite slippery. As I entered a long (empty) private driveway leading into a store parking lot, I accelerated for just a few seconds, and then I applied the brakes. Of course the braking was poor and the car more or less skidded along in a straight line. Everything up to this point was exactly as expected; just having a wee bit of fun, all perfectly safe.

After skidding along for about a second or two, the driver's seatbelt suddenly tightened up (by several inches) by means of an electric motor. The seatbelt was extremely tight, but not too tight. When I released the brakes a few seconds later, the seatbelt loosened up again with the same electric motor sound.

Interesting...