The 'load signal' used for this era of engines is the vacuum, either ported or manifold, which gives you some indication of load. The vacuum level is determined by throttle opening combined with airflow and where the vacuum signal is taken from.
The combustion process is supposed to be a smooth burn, not an explosion. If you get the explosion or any sudden pressure increase, you get the knocking.
When you have the combustion process going on, there is an increase in pressure in the burning cylinder gases, and this pressure rises and falls varies over the burn cycle. There is an ideal pressure increase and decrease in the burn cycle that we would love to achieve that would maximize power transfer from the expanding gases to the piston. But all we can do is approximate that ideal burn cycle with widely varying RPM's and the burn rate characterisitics of practical fuels.
The timing of the spark (advance or retard) is the one thing we can control in the timing of the burn cycle to optimize this rise and fall of cylinder pressure. The classic case of too much advance means that the pressure rises from burning AND also rises due to the piston still compressing, and the result is an explosion, not a continuation of the smooth burn process.
To simplifiy this so you migth easily visualize this: You have to put in your mind that the burn process takes a certain amount of time (it is not instantaneous), and that during the time, the piston is moving too. Proper timing is such that the peak burn pressure occurs after the piston has reach the TDC position and is already moving down again. If the peak burn pressure occurs too early, it will coincide with the piston reaching TDC, and the combination of the peak burn pressure with the piston pressurizing the mixture will cause too high a total pressure and the remaining mixture will explode rather than continue to burn smoothly.
This will happen very easily at low engine speeds and heavy load, like when you have it in too high a gear with the throttle wide open. The low vacuum 'load signal' will cause maximum vaccum advance in the distributor: with too much total advance, the burn process will start but the piston is still moving slowly (due to the low RPM's), and the burn pressure will increase too much, too fast relative to the position of the piston since the slow moving piston still is increasing cyliner pressure as it reaches the TDC point.
The way to handle that low RPM advance problem is to have the mechnical advance set so that it's portion of the total advance is small to none when you are at low RPM's. Which I what you are thinking, if I read your post right.
