Its not a dumb question at all.
Powder bridging/ directional ignition and detonation are all terms that have been used over the last few decades in an attempt to describe pressure problems relative to under loading a cartridge.
If a cartridge is underloaded, the primer flame will burn over the top of the powder coulumn and then down into the powder. In doing so, the optimum burn rate and pressure wave are lost as all of the powder is burnt at once, rather than gradually.
The problem of detonation has been around since the advent of cased ammunition. Any under loaded ammunition can produce these results although some cartridges and powders seem to display greater problems that others.
A classic example is Winchester 296 powder in either the 357 or 44 Magnum cartridges. In this instance, the powder is the culprit and if it is underloaded, pressure spikes occur.
The new super magnums also pose problems, especially the smaller bores. The problem does not occurr as a result of hand loaders deliberately trying to down load. Issues usually arise with starting loads.
In the 7mm RUM, loads below 89 grains (with all brands of powder), can produce severe pressure spikes as a result of detonation. If I want to run 162 to 180 grain bullet in the RUM, the rifle will hit optimum pressure between 94 and 91 grains of Retumbo. As you can see, there is not a lot of room between max pressure and under loaded/ dangerous pressure. The .44 Magnum is the same (with W296), a small window for loading. Winchester give clear warnings about this in their reloading data.
My crappy picture of bridging/detonation/directional ignition.
Again, sticking with the 7mmRUM, the reloader can utilize H50BMG to fill the RUM case to full, such as 102- 104 grains behind a 162 grain bullet. Velocity is now 3220fps to 3275fps and the case is full to the bullet base. The downside of this is the extra heat generated by the greater powder charge- goodbye throat. Remington would have been better off, during the design phase of the 7RUM, to stay closer to the Dakota case length of 2.5"- it would have made no difference to goal velocities.
My next crappy picture- optimum burning.
In my 6.5-55, I can download to nearly half of its usual loading. While this a dangerous excersise, my point is that some cartridges are less forgiving than others with regards to how much you can download. In this instance, I was able to drop 22 grains rather than 3-5 grains in the 7RUM.
If directional ignition/bridging/detonation occurs in a super magnum, the bolt locks up and has to be beaten open with a mallet. Once the case is extracted, it becomes apparent that the primer has been blown out of the primer pocket and that the head of the case is disfigured, losing most of its head stamp. Velocities over the chronograph do not necessarily read higher or lower than usual. I can only guess that pressure is perhaps somewhere above 80,000psi.
One last point. If a 7mmRUM is short throated by 50%, it becomes very dangerous. Using the 162 grain bullet weight as an example. Max powder charge with Retumbo is around 91 grains. Detonation occurs at 88 grains.