strong post -- and good explaination of how it all works ...
one thing i would like to add:
for timing to be efficient it must occur within a set window of opportunity --
when engine speed increases the time it takes the piston to reach TDC is shortened. now if you add a load factor to the speed increase the cylinder pressure must be kept at optimum levels to sustain proper engine output.
if the timing occurs too early in the process (under load) cylinder pressures rise to a unacceptable level -- ie knock events start to occur. now if you add pressurization to the equation (boost) you make the engine more susceptible to knock. simple way to put it --> cylinder volume -> pressure --> start of ignition --> piston speed. with more volume and speed -- time if initial fuel/air ignition is reduced (required) -- a knock event is an indication of cylinder overpressurization and lack of combustion control ..
this is one reason most engine builders/tuners search hard for the best timing under the knock threshold.
so with the increase in load, and piston speed timing is moved backwards to reduce the peak pressure reached @TDC -- having the ability to move (graduated increases) the timing back up to an efficient setting is far more advanced than the old way of power timing (under load till it knocked and then back it off 2-3 degrees).. and believe you me, you never wanted to power-time a forced induction engine -- unless you had a lot of spare parts laying around and nothing to do but teardowns and rebuilds
again -- great post and good information