Wort fermentability can be a confusing topic. In a simple sense, base malts, toasted specialty malts, and unmalted adjuncts contribute long chains of sugar molecules (i.e., starches). The enzymes contributed by the base malt clip chains of sugar molecules of various lengths from the starches. Shorter chains are fermentable sugars, slightly longer ones are unfermentable dextrins.
The saccharification rest temperature is the simplest variable to adjust to alter the percentage of carbohydrates in the wort that will be short enough for brewer’s yeast to ferment. This is because the enzyme (alpha amylase) that works most effectively at the upper end of the standard 140-160°F range produces both sugars and dextrins, while the enzyme (beta amylase) that works best at the lower end of the range produces maltose, which is easily fermentable by brewer’s yeast. At lower temperatures especially, allowing more time for the beta amylase to work also boosts fermentability (if you only mash for 10 minutes at 142°F, the result will not be a very dry beer). Not much controversy there I hope.
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