Master Embroiderer Helen M Stevens tells us about "opus plunarium" which means literally a work of feathers, describing a form of embroidery developed over the centuries. She believes it originated in Anglo-Saxon split stitch. Opus Plunarium is similar to satin stitch but it changes direction so that it gives a three dimensional appearance to the stitching. It is achieved by gradually changing the angle of the satin stitch as you work around a curve or change in shape of the motif, by slipping the next stitch beneath the previous one, about half to two thirds of the way down, so that effectively you have worked half a stitch at the wider end of the motif and none at the narrow end.
I don't have a photo to show you but Helen Stevens does, in her books, which feature the most beautiful embroidery.