Lockdown feature in SharePoint 2010 comes handy when you are using Publishing sites in SharePoint and anonmyous access in enabled.
When you enable Anonmyous access in SharePoint 2010, you have to hide certain Allitem.aspx pages in SharePoint List and Libraries.
To make sure that anonmyous users doesnt access Form pages, we have to enable Lockdown features only through Powershell using stsdam commands. It can't enabled by User Interface.
stsadm.exe –o activatefeature –url <site collection url> -filename ViewFormPagesLockdown\feature.xml
To disable the feature
stsadm.exe –o deactivatefeature –url <site collection url> -filename ViewFormPagesLockdown\feature.xml
Happy SharePointing :-)