That's not easy. I know the trend is towards not using the method where the sergeant yells at his recruits, as seen in the movie Full Metal Jacket. Also, in the military, people feel that the job at hand is extremely important and you can't compromise on the quality of that job. Do you need to yell to get it done? It may not be necessary, but the job certainly requires standards and rigour that leave no room for compromise, and yelling is one way to foster exceptional professionalism.
I believe that's the point of this method. Is it the best method? Maybe there are other methods too. At the end of the day, it's important not to compromise on quality.