Motorcycle Repair: Electronic Ignitions, electrical pulses, dual fire

Question
I have been reading about dual fire vs single fire ignitions.  I believe I understand the differences.  For dual fire I can understand how how a pick up on the crankshaft can cause a spark to happen on each revolution of the engine.  What I don't understand is that if the pick up to generate a spark is on the crankshaft, how does a single fire ignition know if a cylinder is on the compression stoke so it should spark, or on the exhaust-intake stroke, so it should not spark?  Thanks.

Answer
Hi Dave,To put it shortly it is done by electrical pulses at the original time set.All makes vary,though HARLEY-DAVIDSON use a crank position sensor which is a variable reluctant (VR)sensor that generates an AC signal by picking up on the 30 teeth machined in the flywheels.There are two consecutive teeth missing, which are used as a reference point.The (CKP)sensor sends a signal to the ignition module which is used to tell of engine position and speed.