The Flyers entered the game 28th in the NHL on the penalty kill (76.2 percent), but in the past 19 games, they've killed 47 of 57 power plays (82.4 percent).
"The biggest thing is when they enter the zone, we're not giving them any time," said Philadelphia forward Scott Laughton, who played 3:36 shorthanded. "Our defensemen are pressuring on their forwards when they're getting to pucks. Everyone's on the same page, everyone's jumping on them, and we're not letting them set up."
Couturier finished his hat trick at 15:53 of the third period. He kept the puck on a 2-on-1 and shot under Halak's left pad from the left face-off circle to make it 4-2.
"Some nights the pucks find their way in, and that last goal was more a shot-pass and found a way in the net," Couturier said.
Cehlarik, who was recalled from Providence of the American Hockey League on Tuesday, scored his second of the game with 1:06 left to make it 4-3.
Pastrnak gave the Bruins a 1-0 lead with a power-play goal at 4:39 of the first period, and Cehlarik scored from the right circle at 9:12 to make it 2-0.