It's reasonable to assume that something happens in the process of evaporating down to syrup or DME, and that that something differs from a wort made fresh by mashing. From what I've read, it's a combination of sweetness from under-attenuation, and off flavors from the production/storage of syrup. I've not really identified a particular flavor that I associate with it. Perhaps when there were fewer and less fresh options on the market, it was a more common occurrence.
For a pretty scientific hobby, brewers tend to be rather superstitious. Superstitions arise whenever there's unpredictable results from a complicated process over which you cannot have full control (e.g., life). Unfortunately, for most defects and off-flavors, the only explanation or advice, even from experienced brewers, is "Do it again and be more rigorous in your sanitation and temperature control."
Extract twang would seem to be in a different class of defects. It's associated with the major ingredient in your beer, and apparently cannot be got rid of, but only masked. If it's a problem with all extracts, it should show up to some degree or another with every extract batch made from the same lot. If you're brewing with the level of consistency that allows you to taste a consistent off-flavor with every extract batch, you're doing pretty well. If you detect this consistent off-flavor across multiple styles, you might consider the extract as a culprit and include a partial-mash step in your brewing to mitigate it.