Moving maps

Take 7 cards, one of the cards must be key, that is, specially prepared. Before starting the focus display, you need to slightly bend this card. After that, shuffle 7 cards and put them in a horizontal row in a closed form. Then tell the audience: "I'll turn away now, and you have to move from left to right any number of cards, but not more than seven." When the audience moves the cards, you turn and immediately call the number of cards moved. This will be easy as you instantly find the key card.