I had high hopes for this post but theyve been shattered...

A quick description in my own (very drunken) words

Spark timing is expressed in degrees before top dead center (BTDC)

We have spark timing as its necessary to light our mixture and begin the burn cycle at different times with different efficiencies and speeds to achieve our ultimate aim, maximum gas expansion rate at around 15-20degrees ATDC (Top marks Gus )

Due to the fact a given mixture will always take "X" time to complete its burn time, we need to change the time we light it in relation to our engine speed.

Example:
We have a set amount of fuel and air in our cylinder at 3000rpm, and to get maximum expansion we have to light it at say 20 degrees BTDC to get the mximum pressure from combustion to occur at our ideal 15 degrees after top dead center due to the set time it takes this mixture's combustion to progress from being unlit to maximum burn pressure.

So what happens when we have EXACTLY teh same amount and temperature of mixture in our cylinder but the engine is at 6000rpm instead of 3000 as before?

The answer is, we will have to light the mixture at least twice as early as the mixture will take the same amount of time to process in its burn, but our engine is moving twice as quickly!! So to get the same pressure at 15degrees ATDC we will have to initiate the burn twice as early at 40deg BTDC this time...

So what requires a DECREASE in spark advance?
Anything that increases the cylinder pressure and thus raises burn speed.
Examples:
Decreased Air temp. (IF large Density increase)
Decreased Air temp. (Temp threshold)
Leaner mixtures.
Higher Compression.
More air/fuel in cylinder at same air temp.
And much more.........................

The thing to remember is that Spark advance is not something we DESIRE, its something that the engine needs. The higher the compression, the more efficient the engine will be as an air pump. As we decompress the engine we will require more advance NOT because we wanted to achieve this, but because we have DECREASED the engines efficiency and the advance is now necessary to achieve PCP anywhere near 15-20deg ATDC because our mixture burns slower in our decompressed and more inefficient engine.....

Hope this helps now cos i need to sleep.. too much beer guys...