You need to look at diagrams, Google it. Alot of dogs I see are missing just that, 2nd thigh and it bothers me tremendously. Once you see where it belongs you'll always know. It's an easy part of conformation to visualize.
What is called the "second thigh" is the area around the two bones of the lower leg - the tibia and fibula - that are between the femur and the hock. That would be similar to the shin and calf on a human. The length and width of this area are both important to function. Many running breeds (like setters) have a long second thigh in comparison to the length of thigh (femur). This gives them a longer stride through flexion and extension. The muscles of these breeds tend to be flatter, giving a sleek look while adding to the function covering ground efficiently for long periods.
In the retriever breeds in general, the bones of the lower leg are about the same length as the femur - some are even shorter. The muscles tend to be fuller and rounder, which makes the "calf" area look like a second thigh. This is important for power and athleticism. Retrievers do not typically spend hours trotting or running in open fields, but have to be very powerful in uneven terrain for short bursts of activity. A strong, muscular second thigh supports this function.
Technically that second thigh refers to the tibia/fibula bones that are between the stifle (knee) and the hock. All dogs have a second thigh. These bones can be short or very long and affect the turn of stifle/bend of the hock. However, second thigh, in many breeder's terminology also refers to the amount of substance, width or thickness of the musculing behind the actual thigh (femur). I was brought up knowing the second thigh as the amount of substance. I was told by many breeders that this dog or that had good second thigh. It may be that this is what you are thinking of. Some dogs have very little substance in the hind quarters giving them the appearance of a weaker rear. I personally love well developed hindquarters and good substance in the rear.
