These are the terms you need to know. The 'bend of stifle' includes the entire front portion of the leg. The 'point of hock' is the back of the hock where it reaches it's top-most point. The 'tail insertion' indicates just what is says, the point where the tail comes off the spine. The 'croup' is the area on the top from the hip bones down to where the back side of the leg begins.
When you are looking at a dog's rear assembly, their are multiple points to look at. BUT, they cannot be viewed as seperate points. They have to be viewed as one assembly and how it fits together. You will see in picture 1, the puppy has a very nice rear. His hocks are straight and not overly long which is good. This is the same puppy in picture 2, you can see where the line is drawn in picture 2 that shows you where you see one of the angles we are discussing here. This is what is commonly referred to as a 'well let-down hock'. The musculature of the stifle extends itself beyond the point of the hock. This slight difference will enable the puppy to achieve greater drive off his rear at maturity.
Puppy 1 has a better angle than puppy two. Puppy two's musculature at the stifle stops almost at the point of the hock when you draw your line. Puppy 1's musculature extends down beyond the point of the hock. You will see puppy 1 has a much better 'turn of stifle'. When you look at the stifle, you have to look at the entire front of the leg. This should be a nice, deep curve that does not stop until it is beyond the point of the hock. Let your eye travel from the beginning of the front of the leg all the way to the top of the toe. On puppy one, it is a nice long curve. This is what you want to see. On puppy 2, it is not nearly so pronounced nor does it extend as far. Puppy 1 also has a much better croup. It is slightly rounded as opposed to the straightness of puppy 2. The tail insertion is better as well due to the nice round coup. Puppy 1 is almost ideal when you add all those things togther. AND of course this nice rear is topped off by beautiful toes. They are tight and well arched and being so, will give the puppy extra spring in his step. I personally cannot abide a flat foot. If the foot is not a tight, well-rounded foot (cat-foot), the puppy will not ever achive superior movement.
Now visualize that same lines drawn on these two puppies until you can really see the difference. (HINT) Try printing the page and drawing the lines in yourself until you truly understand what you are looking at. This is part of 'developing an eye' for the Collie structure.
On the rough dogs, it does get a little harder without putting 'hands on' and feeling the structure underneath. You can tell some things though. On this dog, you can see he does appear to have adaquate angulation. You can see how deep the bend is through the stifle. What I also see are toes pointing slightly east/west rather than pointing forward. It may be that he is hunkering down, but I would first think he is a little close in hocks and that would cause the toes to point out. You can also see a very nice rounded coup. This dog should have nice extension on side movement, but I would think watching him going away would show his weakness.
First thing I saw in this picture, was a too straight stifle, too long a hock and a significiant drop off of the croup lacking a nice round curve. The toes also appear to be pointing in which means the hocks are slightly pointing out. Again, this dog would lack the necessary extenion on the side movment and no drive or pushing off the rear legs. Going and coming though, he would look very nice based on this picture.
At first glance, this also looks like a very nice rear. Toes are pointing in the right direction and this dog has a lovely curve to the croup. BUT when you look at the angle of the stifle, it is very straight and does not continue though the point of the hock. This dog would lack extension on side movement.
I will leave you with this picture of a very nice rear. There is good angulation, short hock, and all toes are pointing in the correct direction. Looking is not the same as feeling, but this is at least a start. Good luck on your continued education!