Winning a point on the opponent's serve during a tiebreak. In a tiebreak, the server alternates every two points, so holding serve in a tiebreak means winning both of your service points. A mini-break gives you a crucial edge. Most tiebreaks are decided by one or two mini-breaks.
"Mini-break! He wins the point on her serve to go up 4-2 in the tiebreak. That could be the difference."