There are many ways of strength training, a) the muscle sortens and lengthens while applying force like in typical gym weights work out aka isotonic, b) the muscle stays at a certain length while applying force, no0t moving at all, known as isometric, c) plyometrics for eg when you jump a momentary high force is applied while muscles counteract. Also there are plenty of other methods irelevant to what has been mentioned in this topic.
A chained dog that runs and hit is not really under any of the above categories, maybe somewhat under c).
There are experienced dog people that use it, personally I can not see the science behind it, but I always like to learn something new.