I agree! Dwell is the amount of time that current is flowing through the coil expressed in degrees. In a points system the points open on the leading edge of the cam and close on the trailing edge of the cam so increasing the point gap at the peak cause the points to open sooner and close latter (same idea as changing lash on a solid cam). This has a two prong effect opening sooner increases the timing and decreases the dwell.
In an electronic system the reluctor passing the pick-up creates a very short duration pulse that triggers the circuitry in the ecu, it's the delay built into the ecu circuitry that will determine how long the current flow is interupted and hence the dwell. Late model electronic ignitions actually vary the dwell with rpm. They increase the dwell at high rpms for a more reliable spark and reduce it a lower rpms to prevent the coil from over heating. That is why the new stuff doesn't need a ballast resistor.
Adjusting the air gap serves the purpose of ensuring that the trigger signal occurs reliably. To much gap and the signal may not be large enough to cause the ecu to trigger resutling in a mis-fire and to little you could get false early triggers.
FWIW, the only reason that a non-magnetic feeler gage is recomended for setting the air gap is that you can't get the proper feel with a magnetic one because it wants to stick to the magnet in the pick-up. If you are stuck without a brass feeler gage a match book cover is .007-.008" thick and works quite nicely.
