Actually there are two ways to get integrascreen done for EC.
First option is to go through it on your own by contacting integrascreen and get it done and then apply for EC. `
Another option is that VFS will do integrascreen when we submit our EC application to them.
What I had understood is that the second option is no longer an option and from april 2007 integrascreen has become part of EC process.
But the link that you have sent says it is voluntary. Even this link has same information
http://ukvisas.integrascreen.com/DVPBHC ... acker.aspx
"Although the service is voluntary, if you choose to avail of it your visa application will be processed faster, and you will have more confidence that it will be successful. "