To start on the photos, it would be best to fix in your mind, the good examples first. Figure 1, the basic Borzoi dog, is a good example of most points in the breed, and this includes topline. Remember, rising a LITTLE at the LOINS in a GRACEFUL curve. The total correct look is deeply affected by that curve. What goes up, must go down, so croup dropoff, which is not mentioned in the standard, can change the overall effect of the topline.
The black and white dog in Figure 2 also has an excellent outline, though he falls away in the croup a bit more than Figure 1, and is shorter through the back and loin.
In Borzoi, the heaviest parts of the body coat grow in a patch over the shoulders, which in our house is called the "buffalo hump", and over the hips and croup, which we call the "butt toupee". Unfortunately, the heavy hair growth in these two areas, especially in males tends to fill in the curves and puff up the croup. The overall effect is to make the back appear flatter than it really is in a well coated male.
Bitches have the same coat pattern, but it is not as exaggerated. In addition, a bitch tends to blow a lot of body coat after her season, so by the time the two hairpieces have grown out to the distracting length, she usually sheds again. But guess where they shed first..... the middle of the topline. Sometimes this can make a normal topline look almost sway backed! The buffalo hump and butt toupee hold out until the bitter end which is a major frustration to grooming.
So, you say, why not trim it up? Well, many people do, with varying degrees of skill. There are a couple of problems with this. Trimming usually makes the hair feel coarse, especially over the shoulders. Also, not every color can be trimmed. White, or real black, or brindles, can easily be trimmed but so many of the sables and reds have dark tips to the hair and pale roots. Trimming that kind of hair sticks out like a sore thumb.
So what does this have to do with topline? When you are judging, either in the ring or from ringside, you must be careful not to penalize the untrimmed dog for supposed topline faults that have been groomed out of his classmates. The AKC video indicates that trimming is to be discouraged, but there is no justification in the Standard for that discouragement, and everybody does some.
Figure 3 illustrates the first of three very common, very faulty toplines. The dog above has the condition called "wheelback". Her whole ribcage is tilted so that the back actually starts tilting upward from the shoulders. This is a deformity, as it involves an unnatural alteration of the spine, that most important body foundation. Remember "what goes up, must come down?" If it wrong going up, it is bound to be wrong going down. See how the loin never rises. It drops off in an uncomfortable-looking manner, giving her a cramped stance, and see what it does to shoulder angulation. How athletic do you think she will be with a with this structure? Refer back to the skeletal drawing to see what has to happen to the spine to achieve this conformation.
The white bitch in Figure 4 has nearly the opposite problem. First, she is very long bodied, with an overly long loin, slightly forward shoulder placement and a lot of rear angulation. That much length is hard to support unless the animal in excellent muscular condition, in which case there would be more arch to the loin, but she would still have a bad outline. In this photo she looks a bit high in the rear, but you can see that if she was pulled into show stance with hocks perpendicular to the ground, the hips would come down and she would appear even longer. This animal would be more flexible and functional and athletic than the red dog, but obviously not typical.
The red and white bitch above has no rise at all to her loin. Her topline starts to descend before it gets to the loin, the look that the Whippet people call "slap ass". This departure from correct is the most common fault of the three. These dogs are often great sprinters, but lack endurance. Descending at the loin is in direct contradiction to the standard.
All of these faults are both TYPE and STRUCTURAL faults.
The rise over the loin flattens out a bit when the dog trots, a proof of flexibility. Wheelbacked dogs remain wheelbacked at the trot and bounce up and down with each stride.
Flexibility is critical to the galloping dog (or any athlete) as the dog has to bend it's spine in a half circle in the contraction phase of the double suspension gallop and then turn the topline inside out in a concave line as they drive into the extended phase.
While absolute proof of running ability is impossible to determine in the ring, certainly the elasticity of stride, flexibility of spine and strength and muscle tone that give a Borzoi the POTENTIAL to be a great runner can be seen and felt by the astute observer at the gaits available to us at dog shows.