No problem, Jackie - but you're in for it. :) BTW, AWAD is the hobby, I'm a materials engineer by profession.
Metal is metal because of something called dislocations, and how they behave. These dislocations are imperfections in the molecular structure. Metal is strong because it is crystalline (ceramics are much stronger) but they are ductile (tough) because the dislocations can move (unlike ceramics, where dislocations are absolutely pinned in place). To get stronger metal, you have to pin the dislocations, but this reduces ductility and so it is a balancing act. With me so far?
The "favorable stresses" produced by aging require the dislocations to avoid the areas where precipitates are. Instead of moving in a straight line, they are required to go around. In this way, they get tangled up with themselves (work hardening) and also don't move all in the same direction as pushed by the outside force. This distributes the stress in the material and produces less yielding - hence, it is stronger. However, since the dislocations aren't absolutely pinned as in ceramics or "as-quenched" (freshly made, with no further heat treatment) martensite, the metal can absorb shock (this is "toughness").
Favorable stresses are also found in other applications, where metal is bend constantly (fatigue loading). Picture a paper clip that you bend back and forth until it breaks. That is fatigue failure. At the surface, if you shot-peen it, you introduce residual compressive stresses, which resist the bending (tensile) stresses, and improves fatigue resistance to fatigue failure.
Hope that was clear.
Cheers,
Bryan
