What does the crank shaft sensor do?

The Crankshaft position sensor, or (CKP) monitors the Crankshaft position and No#1 cylinder piston position while the engine is running. this information is sent to the engine's ECM. ( Electronic Control Module) the ECM then uses this information to calculate several operating parameters for engine performance but mostly it is used to operate the engine's Ignition system ( coil (s) and spark plugs). back in the day' you could replace your "points style distributor" for an "Electroinic-Ignition system" very much like todays MSD distributors. many of those used a Cam-sensor which essentially performed the same function. some used a Crank-sensor instead. basically:these sensors tell the ECM when to fire the spark plugs. before that it was all done with a set of "points" inside the distributor. hope this helps