The score starts at 18-18. A team needs to win 2 down balls in a row to earn the serve. Points don't get scored until the team is at the service line, and then it's a race to 21.
As Jackie explains, one of the keys to success in this drill is for servers and their teammates to stay poised after working hard to win 2 down balls in a row. Fatigue can sometimes hinder a team's ability to play a strong service point, but grouping together multiple points of steady play during the ups and downs of a match is obviously a huge part of winning.