This algorithm is used to enlarge small images in the Zoomer when Smooth scale option is on. It is faster then bilinear or trilinear algorithm though it is slower then not to use smoothing at all. You can find it suitable for magnifications of very small images like icons though it adds some sort of blur to a picture.
It is easier to compare following two images, representing Windows "Start" button enlarged to 800%, with Smooth scale turned off and on:
Note: this algorithm is not used for enlarging animated images, when Animation is turned on.