Thank you for all the feedback to my last blog, both personally and through facebook.
The answer is that information is communicated via the four remaining cards. It is not obvious how it is done but take a look at the article by Colm Mulcahy, which explains all.
If you are interested in these sort of puzzles I fond this one at Gurmeet Singh’s blog. It is an excellent blog and contains many other puzzles like this.
I was at a conference with Colm in 1999 when we worked out together how to make the trick work and came up with some variations. It felt great to "do" mathematics, even though we knew the answer was out there someplace already. At one point I even had a website that would play the trick, though that seems to have vanished into the ether.
Cheers Mike
It's a small world. Well I guess the world is just as big t'internet makes it seem small.
Nice trick though – it just caught my interest.