several variations
The pre-shadowing gets used a lot in scale-armor circles, minutiae-filled discussions on 'scale-paint' effects, formulae for lightening base paints based on the scale of the piece. etc.
'pre-shading' or 'pre-shadowing' is usually used to darken shadowed recesses, panel lines, grooves, whatever. And when you do your final coats, you work up gradually to your final finish, so that paint doesn't completely coat these recesses. The darker / black pre-coat shows through.
Sort of the obverse of 'highlighting'
Similar to applying a dirty 'wash' (be it muddy thinner, acrylics, oils, pastels, whatever.), excepting that it goes down first, where washes are done after base painting and the solvent the dark pigments are suspended in are used to to transport the pigment to the low / recessed areas.
Have also seen 'pre-shading' down with light colors as well. Shoot the grooves aorund a panel with a dark shade. shoot the central area / raised portions of a panel with a light / white shade, then apply you base color. And stop before the paint shading is 100% obscured.
Really takes an airbrush to have a suitable control over how much / how fast the base paint is applied.
Washes work most effectively when you can wipe down a surface easily afterward, and/or when you want to leave the stain as-is, to be exactly what it is - a fluid stain.
And especially effective when you want a whole subject to be grimy.
Washes are often done with solutions and paints that won't interact with the paints you've already applied. Wouldn't want to take a nicely applied paint job and then hose it off with a fresh application of thinner. Then again, that may be the effect you are seeking (say a white-winter camoflage coat, that's practically worn away).
Another technique is to do your pristine, 'fresh off the assembly-line" paintjob, then clearcoat it for added protection, before rubbing and wiping on it.
I'll round up some samples and throw together a new 'weathering / paint effects' thread on this stuff, maybe we'll get some good info from other experienced folks...