@embeddedt
Sorry, it seems I forget to answer it.
Upscale in this context means to virtually upscale the image to deal with 256x256 subpixels / pixel. When getting the source pixel related to a destination pixel the “upscaled” image with subpixels is rotated. The rotation tells which subpixel of the source image corresponds to the destination pixels. It gives the opportunity for interpolation. E.g. if the very edge of the source pixel needs to be used in the destination pixel then its color is mixed with the neighbor pixel.