It isn't that complicated, gentleman. You all know how points work, correct?
On-off-on-off. Dwell is time in degrees around a circle, remember setting the point gap and then checking for correct dwell? Too small a point gap gives you too much coil on time, the coil can overheat. Too big was no good either. Too big and the coil may not fully charge: weak spark. You needed to adjust the gap just right to get proper dwell.
50% duty cycle just means equal time on and equal time off. The circuit Gordon is using(and just like my original 555 timer) generates a square wave, i.e. on(high) then off(low).
Instead of mechanical contact points, the signal is generated electronically.
Then instead of contacts or a relay, the on-off(square wave) signal is sent to a MOSFET(really a high-current) transistor/switch(instead of mechanical contacts).
Gordon just needs to be careful, because the fan speed of the heater stays the same. I fear that as Gordon increases the fuel(and burner temp!) that the fan won't be able to remove the extra heat generated.