It's just a form of encouragement.
Susur Lee, a famous Toronto chef, doesn't yell at his employees. He just stands quietly beside his manager, Greg Ali, while he does the dirty work. Both claim that
yelling is "part of the culture" of the restaurant industry, and that if Ali screams louder, it's because standards are that much higher. "Good things take time and we need hard work in order to achieve this." Ali also says yelling is an everyday restaurant occurrence, but it isn't personal or racist.
Animal Comment: They yell at you in aerobics class, don't they? And what about those guys who yell, "Stroke!" at the back of those boats?