Barnes & Noble NOOK Color User Manual page 21

Pinch to Zoom
You can use two fingers to shrink or enlarge a book cover or a photo.
Pinch your fingers together to shrink an image and zoom out. Pull your fingers apart to enlarge an image and zoom in.
Pulling your fingers apart this way is sometimes called "pinching out."
Pinching to zoom in or out.
Barnes & Noble NOOK Color User Guide 21