Some magic methods seem so outrageous that the audience wouldn't believe them, even if you told them the secret.  They either have such an elaborate set-up that no one would suspect a magician to go to such lengths just for one trick, or they seem so highly impractical that spectators dismiss the very possibility that it could be the method.  I think the card index falls into the latter category -- that is, it seems like a very impractical idea:  keeping one of every card in your pocket so you can remove the chosen card when it is named.

BUT... that's the beauty of a well-designed card index!  They are not impractical, and in fact, they are not only practical, but easy to use after you give it the practice it deserves.  

The Q5 index design has been credited to Pat Page and Corinda, and it is perhaps the most practical index ever devised.  The entire deck of 52 cards is logically indexed in your two coat pockets.  When you reach for the card you need, your fingers never need to count more than five tabs, and so (once you have mastered it), you can remove a card from your pocket in just a second or two more time thanif there was only one card in the pocket. 

The index is essential for classic effects like Eddie Joseph's Premonition, or you can use it to do a super amazing card to pocket effect.  Imagine having three cards selected from the deck.  After finding two of them, have trouble with the third.  Ask the spectator to name it, and whatever card she names, you immediately remove it from your pocket.  Fellow magicians may scoff and say, "well, it wasn't a signed card.  He must have forced that selection."  But then they'll think about it.... "Wait... I was watching, that was a completely free choice...   hey, how did he do that?"   Of course, lay people will simply be amazed that the chosen card is now in the pocket.

I was in magic for over 25 years before I finally decided to try out a card index.  (I assumed it would be impractcal or too hard to use...)   Well, I use it frequently now.  I wish I'd started using one decades sooner.