To understand why a company called ViSalus is the fastest-growing company of its size in the United States, just watch co-founder 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.