By default, the feature is enabled when plugged in, but it can also be enabled when running on battery in the Settings > Power menu.
During the download, simply press the power button - a window will appear with a choice: continue downloading with the screen off, or put the device into normal sleep. If the user does not interact with the console, it will automatically enter this mode after a certain period of inactivity.
Touching the device will bring up a new status screen showing the download progress. From here, you can either wake up the Steam Deck, or turn off the display and continue downloading.
If the mode is active when powered by battery, the device will go into full sleep when the charge drops below 20%.
The new feature is available for Steam Deck in beta and preview versions.