crankshaft sensor

i have been told that the crankshaft relay sensor is bad on my 97 explorer .what does the crankshaft relay sensor do???

scroll down on this page: http://www.fordscorpio.co.uk/eecv.htm

a descriiption is there. you'd probably find more information on the "cars in general" technical and how cars work sections by searching there...

those are good places to start for any how does it work sort of stuff...