I suspect the pick-up coil(s) (if it has one vs. a Hall Effect Sensor) when excited by the magnet, cause a charge value to be introduced into the ignition module. This charge is then used to electronically reduce the time between firings. This reduction in time artificially causes the plug(s) to fire progressively before TDC. The amount of time reduction would be based upon the amount of charge (which would increase as RPM increased). This type system would always start at retard but it is anyone's guess as to how much advance one would get - especially as components age and values change.