Never done it but seen lots of people asking. Keep seeing this url as a guide:
http://www.wpdfd.com/editorial/thebox/deadcentre4.html
I bookmarked it for a time when I will be attempting something like this, so I will be testing the code at some point.
Someone will be able to give you a more direct explanation, but this could be useful.
Hope it helps you.
Regards,
LC.