In an aircraft mag the peak current out of the AC generator
     is always in phase with the cam shaft and timed around the point where
     the spark needs to fire. That is when the points open
     causing a rapid change in primary current thereby firing the spark
     plugs. The high voltage coil is in the mag with the low voltage coil
     Doing the current generating. It is basically the same as a points and
     battery type ignition system with a primary coil providing the current
     Instead of the battery.

Guys, There are two basic ignition types. Both require a high voltage 
transformer, that is the high tension coil for those
non-sparky types among us. The first type is expanding field. 
the typical magneto is an expanding field device. The spark is
created by expanding the field of the primary, (low voltage side) 
through the secondary or high voltage side. Most capacitor
discharge types (like MSD's) are a variant of the expanding 
field type. The second type is like an old style points ignition.
Collapsing field type. A voltage is applied through the primary, 
creating a field, then collapsed by stopping the current flow.
(the points opening)

 The reason that the mag is preferred is that no external voltage 
source is needed. Mags usually have permanent magnets that
spin within the can to provide the voltage peaks needed. The 
points are to provide a better inrush current at low speed. On a
good mag once off idle the points aren't needed if the magnets 
are correctly timed. One of the old Kawasaki 2 strokes had the
points on the outside of the mag. It was fun to show people how 
at more than 1500 rpm you could hold the points open and the
engine would keep on running! 

Capacitive discharge types as the 
name implies discharge a capacitor through the low tension side
to make the spark. In MSD ignitions they use the super fast rise 
times and multiple capacitors to fire several sparks (as many
as 6 usually) on a single power stroke. The need on fuel dragsters 
is a SUPER HIGH voltage due to the pressure in the cylinders.
The need for better "mags" is the need for enough power to keep 
the spark available at super high cylinder pressure. That is, to
build a better permanent magnet alternator. To all the electrically
 knowledgeable I apologize for this long winded explanation.
Bill Jepson

 
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