The FL mask creates a characteristic spiral shape, leading from the bottom left corner to the center of the image.
Beginning at the bottom left corner of the selection, the FL Mask divides the region into a counterclockwise spiral of 8x8 blocks that continues to the center of the image. Then, the order of this spiral is reversed (the center block is moved to the bottom left, and vice versa). At the same time, the colors of every pixel in the region are reversed (equivalent to a Negative mask).
No rotation or flipping is performed, neither on the scale of the overall selection or of the individual blocks.
The FL Mask leaves a trailing corner in the bottom left of the affected area.