Enable Foreground Mode:
(get-wsusserver).GetConfiguration().BitsDownloadPriorityForeground = $true

Disable Foreground Mode:
(get-wsusserver).GetConfiguration().BitsDownloadPriorityForeground = $false

This has been tested on Windows Server 2008 R2 and 2012 R2 using both Internal and SQL databases.

Thanks to John Weeks for this post http://www.robertskinner.com/2013/01/wsus-on-windows-server-2012-setting.html