First you need to understand the meaning of Sport w/ regards to genetics. What this means according to Webster is a "mutation". From what I understand an unexpected mutation. This term is also referred to in plants, more commonly roses.
More specifically it means to "deviate or vary abruptly from type, mutate".
These mutations stablized, in the crossing of the "sport to a sport". In many cases they do not stablize and the mutation continues and you never know what the progeny will look like.
Some sports do not stablize, hence when you cross one breed with another and then cross their chicks (progeny), their chicks (progeny), may look like one or the other or something inbetween or entirely something else.
This is how they determine a new breed of anything. The children, pups, kittens, etc. etc. must look like both the parents. A Delaware to a Delaware should produce chicks that do not look like either a Barred Rock or a New Hamshire Red. The same holds true for all breeds, or "composite" breeds.