Flap cards are a classic magic apparatus, used to create a color change, but have the drawback of a visible "break-line" at the center of the card.  It is best performed by keeping the card in motion, and performing at a distance.  Despite the drawbacks, flaps are still used today in a wide number of popular effects, not only with playing cards.  The early flap cards were made using internal elastic sheet.  This variation uses a very thin external elastic.