Help with Jim Kassen's decentering test...

On the following page Jim details out his decentering test:

http://blog.kasson.com/?p=14078

I am confused about the proper way to align the target in the four corners.  My understanding is you first center the target then you swivel your tripod head left/right and up/down so the target is flush with the corners.  I assume you don't physically move the tripod or extend/shorten the legs.  Is this correct?  If not how do you align the four corners?

Also, when a target in the corner where do you focus? Do you focus manually or use auto focus?

I'm hoping someone who is familiar with the test or can better interpret the article will chime in.
Thanks in advance for any/all help!