The answer is here in detail

In short

ET ignition is an exploded magneto, Allan has it right, its to do with the magnetic flux peak, also the reason for the ltd advance retard range, stock AR goes out of the juicy spot. ET has a special AR and points cam.

71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod