Balanced structure is really important if you want a dog that can run fast and turn and jump efficiently for years. Badly built dogs will break down in just a few years of work and will have problems running and jumping.
The early part of this article is a little difficult to understand, but the point comes through in the pictures at the end. Once you get a feel for deceiphering structure, it will blow you away how much correlation there is between the fastest, best moving performance dogs and those who just can't do.
My only point of difference with the article is the photo of the chihuahua rescue. The angles are balanced, but I would want to see a little more angulation (especially in front) to absorb the shock of landing and allow the shoulder and arm to fully extend.
The pointer below is fantastic.