Originally Posted by HP_Lovecraft
The twang is produced from maillard reactions during concentrated boils. Full boils, or late boils elliminate the twang.
Some extracts taste "stronger" then others because some purpose mash at higher temps specifically to leave more non-fermentables, and other complex sugars, as well as rich carmalizations. The reasoning is that some extracts were developed with the intention of being mixed with sugars, to offset the thinning effect.
Pretty sure you've got some science mixed up there - maillard reactions are a desired component of many of the complex "malty" flavors that are desirable in styles like doppelbock, and you're right, they do develop in-kettle. Usually they develop as a result of decoction mashing, since it involves boiling drawn-off quantities of the mash. These are the same reactions that are responsible for the browning of bread during toasting - and extract twang definitely doesn't taste like toasted bread.
Extract "twang" is usually attributed to less than fresh extract, as well as canned extract. It's a less exact scientific explanation, but it is where most of the discussion leads.