You turn the brush toward your dog's underside and they stiffen or growl. Stop brushing and give them room to move away. Do not hold them on their back or ask another person to pin them down so you can finish. The belly can be sensitive, and a mat, skin irritation, or pain may make a routine stroke unpleasant.
Ask your veterinarian or a qualified groomer how to handle a painful tangle or irritated area safely. If the growl is new, the dog reacts to touch elsewhere, or they have snapped, get a veterinary check and qualified behavior guidance before trying again. Do not cut mats close to the skin yourself, it is easy to injure a moving dog.
Rebuild the routine in small pieces
At a calm time, show the brush without using it and reward your dog for staying relaxed. Put it away. Later, offer a brief touch on a comfortable area and stop before they show tension. Let your dog move off and choose whether to return. Only when these steps are easy should you approach the belly, perhaps beginning with a hand near but not on it. Progress may take many short sessions.
Use an appropriate brush for their coat and check for pulling. If grooming cannot wait because of a medical problem, ask the veterinary team for a safe plan rather than practicing through pain or risking a bite. A complete grooming session is not the measure of success today. A dog who trusts that you will stop is easier to care for later.
Sources: Dogs Trust: How to handle your dog. Dogs Trust: How to understand your dog's body language.