Malt modification and stewing before kilning are unrelated, but are both clearly part of your question. The term modification is used to describe the changes that occur when barley is transformed to malt, and one of the key indices of modification is the ratio of soluble protein to total protein. This value is called the Kolbach Index, and is frequently listed on malt specifications as “S/T”; Kolbach values in the 40–45% range are normal for well modified malt, values of 35–40% are typical for “lightly modified,” and values less than 35% are typical for undermodified malt. Free amino nitrogen (FAN) is related to S/T, and is a measure of how many protein fragments (polypeptides and amino acids) are present in wort. Normally, modified malt has a FAN level greater than 180 mg/L. Brewers like using well-modified malts because starch conversion and extract recovery is made easier.
Crystal malts are made by adding a step to the kilning process called stewing. Some crystal malts are stewed on the germination floor by keeping the moisture content during the early stages of drying high, sometimes by covering the malt with something akin to a tarp, and other crystal malts (really the majority) are stewed in drum roasters. The stewing process is essentially mashing in the malt kernel before malt kilning, and this process creates fermentable sugars and free amino nitrogen from malt starch and malt protein. These two broad classes of compounds are the basic ingredients of the Maillard reactions. As the moisture content is reduced after stewing, and the grain temperature increases, the simple sugars and free amino acid components react; the result is color and flavor.
Pale dextrin malts are not kilned at high temperatures, and therefore have minimal color and flavor from Maillard browning. The particulars of dextrin malt production are guarded by the companies who make these special malts. Regardless of the method of production, malts with normal to high FAN levels can be produced from undermodified or well-modified malt when stewing is used since stewing is not much different than mashing. The same is true with wort; normal to high FAN levels can be achieved in wort using undermodified malt, provided that the mashing conditions favor the enzymatic degradation of proteins/polypeptides by proteolytic enzymes.