Whatever. You don't understand math and physics either until you can explain it to your grandmother. Einstein said that, and I figured it out myself trying to explain it to my wife. Complex ideas in nature usually have simple and elegant answers once you understand completely. Which is good, because most forums aren't conducive to displaying the mathematical formulas necessary to model these systems.
Look, when you decrease some powders, the powder burns and builds pressure at different rates compared to others. They are all different and all experience this to some degree, but it is most apparent in slow powders when used in firearms. So if you have, say a slow burning powder, and you reduce it, it will burn and expand and build pressure until it begins to push the bullet, which only requires so much pressure to get moving.
If you have the case full of a powder like H110 it will burn, expand, reach the appropriate pressure, then push the bullet.
If you have a case half full of H110 (for instance) it will burn, expand, but instead of reaching the appropriate pressure to push the bullet in a nice predicatable fashion, you get a pressure spike. It expands to fill the space, but too fast. This creates the pressure and thus the problem. It'll push the bullet, but not before the pressure jumps to a very high level that can exceed the ability of the chamber to hold.
But all isn't lost. If you "work up" your loads, you can "work down" too, but you'll need a chrono to do it best. You can't really "feel" it, although some will feel snappier than others. When you get into an unsafe area, you may not know it, but with the chrono, you'll start seeing the numbers get erratic. This is indicative of pressure spikes when using reduced loads. If you have 50fps difference, I don't see that as a problem indicator personally, but a 200fps spread is. Scale back up until you get a predicatable spread. Note that temperature will affect these numbers too, hotter gives higher pressure and cooler lower --whereas it may not be important otherwise, when dealing with max and min loads it is. Always.
This issue of pressure spikes seems to pop up most often when using low explosives in a sealed container. If the charge fills up the case, you have less problems. Half full cases of low explosives are, in fact, necessary for high power explosions in absence of high explosives. Keep this in mind, it is a rule of thumb. Because low explosives aren't very brisant, a pressure spike is desired when using it in demolition --but avoided when handloading. Remember, handloading and demolition are two different things, but some handloaders seem to confuse the two, thus KB's, which can come equally from low and high charges.
Also note that when loading subsonic ammo, you get the same problem. If you load a rifle cartridge with enough rifle powder to get subsonic velocities, you get erratic velocities and sometimes KB's too. So subsonic load data is usually scarce, and they are usually loaded using pistol powders --because they will fill the case more.
An option is to use a filler, but I haven't messed with it. A filler will prevent the pressure spike by reducing the case volume, theoretically you can use this with any load. But in practice, settling occurs, and the whole thing is dynamically modeled, so at some point you will reach a point where there is too much filler to give reliable ignition or sufficient pressure.
Another low pressure event is caused when the charge is too low, the bullet gets stuck, and then the pistol blows up. But step back and you will notice that the same thing is happening --the bullet goes so far, plugs up, but in the process creates a larger combustion chamber allowing the gas to expand and a pressure spike to occur. In this case, the pressure spike occurs before the slide reciprocates and you get the KB that way.
Bottom line: you can do it safely, but understand what you are doing. Best advice is to go with a minimum charge load using a powder that fills the case as much as possible, ie, a high volume powder.
Hope this helps, answers questions, and dispells myths.
