You Snap, Flick, or Wave at/on the card and the mysteriously changes.
What You Need
What you need to do the snap change are 2 ordinary playing cards that "merged" into one. Audience must see and think that you only hold one card.
How To Do - Step 1
The Fingers Position
Hold the cards with "the front side" facing your audience. If you are right handed then hold those cards with your right hand.
Put your thumb on the back of the cards and your other three fingers (the index finger, middle finger, ring finger) on the front of the cards. Your middle finger position is an important factor.
How To Do - Step 2
The Movement
Basically you just withdraw the card on the front (the one that audience see before the movement) to the back and make the card on the back become the only card that is visible to your audience. Use your middle finger to bring the card on the front to the back.
Note: In Stead Of Flicking The Card, Wave Or You May Snap Your Fingers As Well And If You Have 2 Of The Same Card But Different Color You Can Do A Color Change. Keep Practicing.