sensorless electric motor position detection

[Dave Zug]

I was recently at a trade show (industrial process control equipment) 
and a vendor had an interesting display..
It was a wheel that you could rotate and a readout that showed position
using the signature that the magnet poles made as they spun. I proceeded 
to rotate the wheel 180 degrees and behold! a reading that was 180 
degrees in error! i really didnt expect to fool it, i also could trick 
its position calculation by quickly passing a mark, then returning it to 
that mark.  Novel idea, needed more time to complete but looks possible 
if you can overcome the detection of which magnet winding you were 
looking at. aparrently its cheating if you manufacture a flaw into a 
motor winding on purpose. 
I thought you thinkers would be interested in this.

anyway, can someone tell me how ABS detects wheel position/movement?
(keep this one for the FAQ!)

and, is ABS considered EFI related? (be nice)