When using a card scraper, wrap three fingers around each side and push your thumbs into the opposite side of the scraper to bend the plate. To begin working the wood, lean the scraper forward and press and push through the base of the scraper plate. The more you bend the card and the more force you apply, the more material will be removed. As the card scraper takes off so little material at a time, it is not obvious after the strokes are taken and shavings removed. The enables you to focus on a precise local area to remove any defect without worrying about creating a concave in the surface. If you are just removing dust rather than shavings, the card scraper is probably in need of sharpening.
Wood surface before and after using a card scraper: