Sorry, I'm not really explaining the effect very clearly. This is largely because I've just explained the gimmick used, and the effect is very straightforward and clear cut. Thus now explaining the effect would, I feel, be giving away far to much information as the trick is currently being sold by a very well known mentalist on penguin magic.
I guess all I can say is that it's an on going effect, not just a *bang! there is your card* effect. As such at the start of the routine the most obvious method of doing the trick is to use marked cards. Thus the magician in question suggests giving some of the cards to spectators so they can check the backs for marks. After this the trick continues with a *ramped up* variation.
Wouldn't it therefore be easier to use a marked deck?
No, The card to be identified needs to be at the bottom of the pack, thus ruling out the possibility of any visible markings. The markings I was referring to earlier where kinesthetic markings, ie coating a card in roughing fluid or something similar.
I realise everyone reading this will now be shouting at their screen "what kind of lay person would suspect you of coating the card in roughing fluid?!" Again, I can only say that it's the obvious conclusion when you see/hear of the effect which I can't say for reasons mentioned above.