To understand why a company called ViSalus is the fastest-growing company of its size in the United States, just watch cofounder Nick Sarnicola in action at one of the company’s periodic sales conferences. In the video that ViSalus posted on YouTube of a July conference in Miami, Sarnicola’s turbocharged pitch inside a packed 18,000-seat arena has people on their feet, pumping fists, clapping, waving, even dancing. A politician or entertainer can only dream of an audience response like this. People don’t usually pay good money to travel to Miami in sweltering summer heat and then readily wedge themselves into a packed arena to see someone strut around and talk on a hastily assembled theatrical stage — about a company whose major product is powder for a weight-loss shake. Sarnicola is an unlikely standard-bearer.