Great explanation I would only like to add how that hose valve You talk, is opened so the spark that ignites the fuel can be generated, Thistask is performed by the the pickup coil, a little magnetic sensor mounted very close to the magneto/rotor .The purpose of this coil/sensor is to send a voltage signal to the cdi module (also known as ECU) for spark to ocurr. Everytime the magneto/rotor tab(s) passes by the pickup coil ,there is a variation in pickup coil magnetic field producing a voltage change going the ECU , this voltage variation is also known as Hall effect. usually on the 0 to 5 V range .The ECU receives this voltage and sends the ignition coil a signal to generate a spark at the designated cylinder. But this is very curious here, because that signal instead of beeing a voltage is just the opposite, It is the suppresion of voltage sent to the ignition. coil primary winding that forces the magnetic field inside the ignition coil to collapse ,thus producing a high voltage and a spark in the secondary winding which is connected directly to the spark plug. In other words the signal sent from the pickup coil to ECU/CDI grounds the ignition coil primary circuit , making the already existing magnetic field inside the ignition coil to collapse, creating a very high voltage inside the secondary winding ,(usually in the 40.000 V range), which jumps to ground via the plug and so the spark is created.
Pickup Coils are also known as transducers.These elements convert one type of energy (in this case magnetic) into another (commonly voltage variation).Transducers are very useful to measure different variables like position, temperature,light,proximity etc.beeing the more common sensors used in vehicles for their versatility and simplicity.