You would start with the widest and deepest toothed carding knife and successively use finer and narrower models (just like sandpapering) to get the finer undercoat or in shorter more sensitive areas (i.e. head). You know when to stop working an area when you can no longer feel or see longer fuzzy undercoat and the outer guard hair is laying flatter. My favourite quality brand is the Mars Stripping Knives. They last forever, have a wide variety of sizes, work well in tight areas, do a great job with out harm or damage, and the pets enjoy it. They are less expensive than the FURminator.
The FURminator has a thinner blade, and therefore a sharp edge, with tiny teeth. It is designed in the form of a rake, with the handle perpedicular to the cutting edge. Yes I said cutting, as this tool can cut and scrape. The handle design being perpendicular encourages the user to drag and use too much force in order to stay in contact with the hair since the teeth are very tiny. The action means the blade is dragging at a 90 degree angle to the hair, so it is catching and shredding the hair, not combing.
The FURminator is deceptive because it looks like you are removing a lot of hair, but what actually happens is you are scrapping and shredding the outer coat and undercoat hair along the top dorsal, with little focus on the areas that actually have more undercoat, like the hind end and mane. Because the FURminator is indiscriminate by design in which hair is being removed, a user doesn’t know when to stop because hair will keep coming off until the damage has been done and bald/damaged areas are apparent.
I can always tell immediately when a pet’s owner has been using the FURminator. The hair is literally shredded and the hair cuticles damaged. Because of this, these pets tend to mat more easily, as ragged cuticles absorb more dirt and snag on one another. Underneath, the skin is red and scrapped. Sometimes the damage is so bad that only alternative is to trim off the damaged hair, throw away the Furminator, and start over.
I teach a lot of grooming, and my first rule for newbies is to use a grooming tool on yourself first to understand how it works, feels, and how much pressure or finesse to use. If it is a tool you wouldn’t want to use on yourself, chances are you shouldn’t on a pet. Keep in mind human skin and hair follicles are twice as thick as a dog, and a dog’s skin and hair is twice as thick as a cat. A cat’s hair and skin is a 1/4 as fine as humans which makes them especially prone to tearing and damage.
If you want to keep your expensive FURminator investment, I suggest you dull the edge, use minimal pressure in the dragging action, use evenly all over the body, and use frequently for a very limited time.