I do know that magnetic pick-up coils are used to detect crankshaft position and these sensors are used to tell which cylinder is misfiring. These magnetic pick-coils do not produce a smooth & consistent sine wave when there is nothing wrong with them. Also... as far as I know this is the only sensing mechanism that OBD II uses to isolate a misfire to a particular cylinder. When I have changed a faulty magnetic pick-up sensor it was for intermittent and random cylinder misfire and I was able to fix the problem. When a particular cylinder has a repeat fault code for that same cylinder and the engine runs fine, a leak down test shown to be within factory specs, pressure test of crankcase was within factory specs, pressure testing of intake manifold shown that there was no leaks and pressure testing of cooling system proved to be within specs and from time to time as well as over the coarse of two years this same machine produces a error code for that same cylinder. There is "no" Issues as to the running performance of this engine. Just do to the fact that this machine runs fine I want to make sure that the only sensing mechanism is the crank shaft sensor that OBD II uses.
The misfire detection monitor runs during normal engine operating and driving conditions and is used by the PCM to determine whether the engine is malfunctioning to the point where it is releasing excess pollutants into the atmosphere. The OBD II system detects misfires on most vehicles by monitoring variations in the speed of the crankshaft through the crankshaft position sensor.
A single misfire will cause a subtle change in the speed of the crank. The PCM tracks each misfire, adding them up and averaging them over time to determine if the rate of misfire is high enough to cause the vehicle to exceed the federal emissions limit
There is another website explaining how OBD system check the misfiring: