A good readable explanation Can - thanks for clarifying 
I will share my simplistic thought process on timing as it may help others who are similarly handicapped J
The piston comes up towards the combustion chamber and when it is a top dead centre this is 0 degrees.
If the spark fires before the piston hits TDC then this is advanced timing and because the piston is still “squashing” the mixture upwards as the spark is fired it makes it a more efficient burn as more fuel/air mixture can be squirted in the greater volume (compared to TDC) which is there.
The retarding bit is a bit more simple, simply the spark fires when the piston has gone past TDC and is on the way down. Once the piston is on the way down the compression ratio of the engine (ie how much “squash” the mixture had) rapidly diminishes so for example if there was too much boost (on a fixed boost explodamotor for example) the ECU lets the piston drop past TDC and fires the spark when the compression ratio (squash) drops and this makes it a safe burn. It is this high compression ratio (brought about by too much boost for the static CR) which in combination with heat needs the combustability of the 98RON+ otherwise the knock will occur.
I understood there were limits to how far the ECU can retard from the furthest advance setting, (I think it was 7 degrees on the 993tt engine) so the further advance you go the less retard is available and with the tuner engines the retard available is not enough to cope with the lower RON fuels…

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2009 997 GT2 RS Tuning 542PS/736NM