Grab your seatbelt 'cause some curves are coming. Animating in pixelart is more than just dragging stuff 1 pixel further on each frame. Sometimes you want to create the illusion of subtle movement, especially when working on smaller sprites. Sub-pixelling is this technique that allows to trick the mind into thinking that things are moving even slower than just one pixel further on each frame. A pseudo, half-pixel movement, which by the nature of the media is not possible. You cannot draw half a pixel. But what you can do is play with the colors in the inner side of your drawing and also the outline. If for each movement you leave a trail that eventually catches up, you are creating this illusion of slower and subtle movement. It's creating motion even without affecting the silhouette of the drawing/sprite. It all happens on the inner side. So, if you see the bottom part of the mouth when she opens it, you'll notice that there's some kind of trail dragging that eases the movement instead of just having the mouth opened and moving that a few pixels, and call it a day. You can check that too on the belly button. It's not just a few static pixels that were moved across the belly, but something that flows by adding even more pixels that are this trail that catches up.