Wheel rotation pickups and torsen mechanism

All,

Saw something interesting on my car when it was in for the tire install
yesterday.  On the axle just inside the rear brake rotors on both wheels,
there is what appears to be a series of magnets, and a magnetic pickup
hanging over them--some sort of electronic rotation sensor.  Possibilities
that occurred to me are 1) speedo pickup, but I thought that's usually on
the transmission, 2) wheel rotation pickup for the ABS, but they should be
on the front wheels too, and I didn't see any similar thing on the front,
or, the only other thing I can think of is 3) wheel rotation pickup for the
LSD.

This got me to thinking about how the Miata's LSD works.  The descriptions
I've read about generic torsens is that they work entirely based on an
internal mechanism, with no electronic feedback, that uses worm gears
instead of hypoid gears so that transmission of force is unidirectional from
the differential to the axles, and not from the axles to the differential.
Does the Miata torsen actually have axle rotation sensors, with a
clutch-type LSD where the clutches are controlled by electronic feedback
loops?  What gives?

    Eric Lucas