This is a really neat deck. Every other card in the deck is alittle shorter than the rest. For example: the first card is shorter, the next card normal, the next card shorter, the next card normal, and so on throughout the deck, Usually, the shorter cards are all one(in my deck they are the jack of clubs) So if you riffle the cards facedown, you'lll notice that the cards fall down by twos.(two cards each fall) The shorter card will always fall on top. When you riffle facedown you will see the bottom cards before they become the bottom cards. The bottom cards are normal, therefore the deck appears to be a normal on. If you riffle the cards face up, then you can only see the top cards, which are the shorter ones. If you don't have one, this is a good deck to force a card, and even to 'magically' transform the deck into a deck of nothing but one type of card.
-From Justin Johnson
The Svengali Deck consists of 26 ordinary cards, all different, and 26 short cards all of the same suit and value. The latter may be narrower as well as shorter, but short duplicates only are generally used. The pack is set up by arranging the two sets alternately, thus every other card from the top is a card of the same suit and value. Burling Hull in his "Sealed Mysteries" claims its invention and that he copyrighted it in 1909. The Svengali deck soon leaped into wide popularity and into the hands of street peddlers. Many thousands of packs must have been sold, and are still selling, and yet its use must not be despised by magicians on that account. Like many other weapons in the magicians' armory it can be used even amongst people who know the principle without their suspicions being aroused.
-Encyclopedia of Card Tricks
Basic Ways of Handling the Pack
After giving the cards a riffle shuffle, which does not disturb the arrangement, square the deck and hold it face down with the outer end slightly raised towards the spectators. Slowly ruffle the cards by placing the tip of the right fore-finger on the outer edges of the cards, bend the pack slightly upwards and release the cards rather slowly, every card will be seen to be different. The short cards do not, appear since the cards fall in pairs.
Lowering the pack, again ruffle the cards and invite a spectator to insert his finger tip, at any point he desires. No matter where he does this his finger will rest on the back of one of the short cards. Divide the pack at this point, let the spectator take out that card and the force is made.
It will be noted that the bottom card of the portion lifted off with the right hand is an indifferent card as is also the top card of the left hand portion after the card has been withdrawn, by showing these cards the apparent fairness of the choice is established.
From this simple many astonishing effects have been developed and it is safe to say that there are possibilities of further card miracles waiting to be evolved by ingenious minds.
The greatest effect is obtained by switching a deck that has been used for several tricks, in the course of which the spectators have freely handled and shuffled the cards, for a Svengali deck. Results can then be obtained which to the layman appear miraculous.
-Encyclopedia of Card Tricks