The anal glands also called annal bags are located on either side of the anus. They have a strong odor that allows the dogs to recognize each other.
Generally, the annular bags empty themselves when the dog goes to the saddle. Large dogs rarely need human intervention for the maintenance of the annular bags.
I strongly advise against systematically emptying the anal glands of dogs of small breeds. For some dogs, emptying the anal glands can prevent an infection. An infected gland will be swollen, sensitive and warm.
The glands are presented in several colors and textures (liquid, pasty or granular). Improper handling can cause serious damage. If there is infection, consult the veterinarian.
Too frequent emptying of the annular bags stimulates and causes the anal gland to secrete much more pasty liquid, which results in a dependence on this action. It is normal for a dog to sled, it is not this action that empties them.