Oops, flip those 2 around. Mashing low activates the beta amylase which makes more short chain sugars which are more fermentable leading to drier beer. Mashing high deactivates the beta amylase quickly and favors the alpha amylase which results in more long chain sugars that aren't as fermentable leading to a more malty, full bodied beer. At 170 both alpha and beta amylase are quickly destroyed and all conversion stops. That's why people who fly sparge will raise the mash to that temperature, to stop the conversion where they want it.
I think I mis-spoke. I bring the water to about 170 and turn the heat off. Quickly drops to about 155 or so once I put the grain bag in.