Mashing, the conversion of grain starch to malt sugars is accomplished by 2 enzymes, alpha and beta amylase. These enzymes have specific temperatures where they work and they differ in how they work. Beta amylase makes short chain sugars that the yeast eat quite readily but it works at the lower end of the mash temperature and is denatured as the temperature goes up. Alpha amylase starts working at a higher temperature but it doesn't break down the starches into fermentable sugar. When you mash low (146-150) beta amylase is the most active and you will get beer that ferments out to a quite low FG, I've seen one of mine end up at 1.002. As the temperature goes up there will be a mix of sugars produced until the temperature is high enough to denature the beta amylase. At 158, the beta amylase would not be able to function for long so you get unfermentable sugars. These lead to beer that is "malty" and the FG will be higher than wanted.