He draws historic domestic spanking and school corporal punishment scenes. The spankings are always given bare bottom (sometimes in a state of complete nudity, sometimes with nudity from the waist downwards, but at least with the pants and underwear lowered to the knees) with an open hand, a hairbrush, a birch or a cane. The spankee characters range in age from preschool to teenagers.
Riis draws in line art style, somewhere in between cartoon and realism. His artwork is usually black and white, but there are also many colorized versions of them around. Most, perhaps all of these seem to have been colorized by fans and not the original artist, however.
Due to Riis not signing his work it is often hard to definitively associate them.