Per an ESPN Insider article (only posting 2 paragraphs):
If we're being true to the role of a closer in baseball, we're looking for the player who takes matters into his own hands and increases his usage the most with the game on the line. To measure this, we'll call upon the usage rate statistic, which calculates how often a player uses a team's possession while on the floor either by field goal attempt, free throw attempt or turnover. And how do we define when the game is on the line? For these purposes, we'll adopt NBA.com's definition of clutch situations: when the score margin is within five points in the final five minutes of the game.
There's a graph in between that illustrates the stats that the "Holy smokes" is in reference to.
Holy smokes, Kyrie Andrew Irving. It's a shame he's been shut down, because the kid has wasted no time to assert himself on the big stage. The 20-year-old rookie leads the NBA in added usage in clutch situations. He normally uses "just" 27.9 percent of his team's possessions, but when the going gets tough, Irving gets going. In clutch situations, Irving raises his usage rate to 47.2 percent, which is tied for the highest in the NBA along with Carmelo Anthony. And he's not just chucking up shots like Anthony; Irving has made just as many crunch-time shots as Anthony in 28 fewer attempts.
82games.com for the whole picture: