I don't know whether this will help, but I'll offer it up. I don't have this effect, but Alakazam just now sent me a promo email about it and The Grail. I had a look at the video:
http://www.alakazam.co.uk/demos/ZENNER_ADVERT_NEW.wmv
which, from the title, I'm assuming is a new version (I haven't seen any other one). I nodded to myself and said, 'Ah, yes, I see. Clever.' The method seemed pretty 'obvious', but the effect is nice. I then glanced at TM to see if anyone had talked about it. (I think the chap who suggested writing on the back of the cards with a Sharpie rather than using those fiddly, distracting and unnecessary stickers was right.) The thing the performer does at the beginning might have been introduced in this demo to address the problem of 'the suspicious initial deal' people were mentioning, I don't know.
Anyway, if you haven't seen it, watching this performance - and knowing what you don't see in it - might help clear up whatever is puzzling you.