They're actually making mochi, which is a rice cake made from cooked and then pounded (as seen in the video) rice flour. The traditional way of making it involves 2-3 people. 1-2 pound the rice cake and 1 person turns it between pounding. Those who have been making it the traditional way for a long time get a really fast rhythm going. You see this often in more country-ish spots and touristy areas. Mass produced mochi is made by machine now.