During mashing pull a third of thicker mash to a separate kettle. The majority of enzymes will remain in the main mash. Then do an amylase rest at 158F for 20 min for a quick conversion, then bring it to a boil. For light beers boil for less time <20 min, richer heavier beers for longer. Then return the boiled decoction to main mash to reach different rest temps. There are different versions and methods all the way up to triple decoction!
– Brewmaster Jeremy Gross