Scott Girvan said:
Chalks over paint would be more like post-shading.
From what I've seen it painted free hand. You simply painting a stripe along the panel line.
It seems like there's not generally a clear consensus on what is pre-shading and what is post-shading...
I think the key idea of pre-shading is that the shading is applied underneath your main color coat. As a result, you can create your color variances such that they're very clear and vibrant, and then when you lay on your main color, all those stark color differences are massively muted by the main color layer, but still present.
Post-shading would be any technique where the shading goes on after the main color coat: pastels, post-shading with the airbrush, various uses of filters, washes, dry-brushing, etc...
Among Gundam builders there's a popular variant of pre-shading known as the "Max Technique" - named for Max Watanabe, one of the modelers who popularized it... Basically it's a set of color layering formulas to produce different effects. Most of the colors are produced by using translucent paint over a shaded under-layer. In this technique it's also more common to produce the shading by starting with the darker color, filling the panel, and then building up the lighter color starting from the center. This tends to use more paint but it's an easier technique to control: and then since you're layering over the whole thing with a (possibly translucent) overcoat, the mistakes that were
barely visible during the shading process become even harder to spot, while the starker shading of the under-coat becomes more muted and subtle. I've been moving away from use of this technique lately, trying to build my skills in other methods of introducing color variance and subtlety to a paint job - but that sort of approach can do a lot to give a paint job depth...