In either case, the puppy or dog will use his arm strength to pull his body forward across the floor.
In most cases, if it is very brief behavior, done at random times, it will be harmless. Though, with other dogs, if this is done in place of walking or happens quite often, this is a red flag.
This behavior may mean one of several different things, depending on the details:
1) Does he show any signs of having trouble walking? Is there a limp? Does he favor one leg over others? If he is scooting on his stomach to avoid walking, this would point to a leg/hip issue such as luxating patella or hip dysplasia. This would need the immediate attention of a vet.
2) Does his stomach look irritated? Does it have pink or red spots or bumps on it? If so, he may be scooting on his stomach as a way of 'scratching an itch'. This would point to possibly some sort of allergy that is causing itchy skin or it may be just a case of a dry skin problem.
Allergies often manifest as skin problems
including dry skin
- most often seen in the winter when the air is very arid - and this can cause cracking, peeling and itchiness.
While it would normally cause itching all over the body, another possible reason is fleas. They are notorious for hiding in the groin and armpit area of dogs and can cause a dog to behave oddly to find relief. Aside from rubbing the body across carpeting and other surfaces, a Shih Tzu will often shake his head since fleas can cause quite a bit of irritation to the ears.
If fleas have just attached to your dog, you may not see them yet. However if you carefully part the coat and look closely you may see the telltale small black or brown specks.
In any case, this would need immediate attention of a vet to determine the trigger and treat the rash/bumps/hives or other cause of the belly area being so itchy that your Shih Tzu is trying to scratch it.
3) Does he behave in an amusing, playful way when he does this? Does he try to gain your attention, acting happy and looking right at you? If so, we'd suspect he rubs her abdomen on the floor as a form of play and to urge interaction. Some dogs do a sort of 'army crawl' just to be silly. If owners behave amused, they now have incentive to do it over and over, just to bring about smiles and chuckles.