The Mets then went out and provided two good examples.
Jose Reyes' game-tying two-run homer in the ninth inning and Asdrubal Cabrera's walk-off three-run shot in the 11th fit the bill, lifting New York to a dramatic 9-8 victory over Philadelphia at Citi Field and boosting the club's Wild Card hopes.
The idea behind the barrel is that it takes a combination of high exit velocity and the right launch angle to produce a batted ball that is likely to do damage. To qualify as a barrel, a ball must be hit at a minimum of 98 mph. At that exit velocity, it must have a launch angle between 26-30 degrees, but as the velocity rises, the range of angles expands.
The metric is designed so that a barreled ball is one whose combination of exit velocity and launch angle carries an expected batting average of .500 or better and an expected slugging percentage of 1.500 or better, based on past results. But the overall average and slugging percentage on barreled balls are .800 and 3.000, respectively.
Not surprisingly, the Mets' leader in this category is Yoenis Cespedes, who has collected 34 barrels this season, including one on a fourth-inning single on Thursday.
While Cespedes entered the day producing a barrel in 6.6 percent of his plate appearances -- well below Major League leader Khris Davis (10.8 percent) -- Reyes was at 3.4 percent (eight total) and Cabrera at 3.6 percent (19). But both infielders delivered one when it mattered most.
Reyes came to the plate with one on and one out in the bottom of the ninth and the Mets trailing, 6-4. Phillies closer Jeanmar Gomez fed him a changeup, and Reyes was all over it, connecting at 103.2 mph and at a 23-degree angle. Batted balls with similar traits have gone for a hit 89 percent of the time and a home run 48 percent of the time this season.
This drive soared over the right-field wall, landing a projected 397 feet from home plate and ultimately sending the game to extra innings. It was Reyes' eighth homer of the year, his first with a runner aboard and his first to come later than the fourth inning. In fact, it was the first time Reyes had gone deep in the ninth inning or later since Aug. 26, 2012 with the Marlins.
That clutch performance set the stage for Cabrera, but not before the Mets fell behind by two runs once again. Now trailing 8-6, the Mets got a walk from Michael Conforto and a single from Reyes to bring up Cabrera against right-hander Edubray Ramos with one out in the 11th.
Cabrera, who finished the game 3-for-6, entered the day batting .358/.418/.688 with eight homers in 122 plate appearances since returning from the disabled list on Aug. 19. Ramos made the mistake of hanging a slider to a hot hitter, and when Cabrera walloped the ball at 102 mph and 29 degrees, he flipped his bat in style. Similar batted balls have been hits about 79 percent of the time and homers 63 percent of the time this year.
"As soon as I hit it, I knew it would be out," Cabrera said.
Indeed, the ball cleared the wall in right for a projected distance of 393 feet. It was only the third time since the start of last year that Cabrera homered off a slider, and it was the fourth walk-off blast of his career. Of the switch-hitter's 22 big flies in 2016, 19 have come as a left-handed batter and 17 at home.
This one set off a wild celebration as the Mets, thanks in large part to this pair of barrels, remained tied with the Giants atop the National League Wild Card standings.
Andrew Simon is a reporter for MLB.com. Follow him on Twitter @AndrewSimonMLB. This story was not subject to the approval of Major League Baseball or its clubs.