via youtube.com
Scooting... set to one of my favourite pieces, Bach's Cello Suite 1: Prelude - a lovely piece, think Master and Commander theme. Possibly a little ruined forever by the theme of the images!
If your dog does this you are probably amused - or irritated when they leave brown smears on the floor! However, you really need to get it checked as it most often caused by inflammation, infection, or impaction of the anal sacs.
Of course, it might be fleas, or tapeworm segments, or an inflamed bowel, or a tumour..... but worth getting it checked :)