Goon

Doug Glatt (Seann William Scott), a bouncer at a bar in Massachusetts, feels ostracized from society, especially since his father and brother are both successful physicians. Doug attends a minor league hockey game with his best friend Pat (Jay Baruchel). Pat taunts the visiting team during a fight and one of their players climbs into the stands, calling him a homosexual slur. Doug, whose brother is gay, steps in and easily beats up the opposing player. Soon after, Doug gets a phone call from the coach of his hometown team who offers him a job as an enforcer.
Meanwhile, veteran enforcer and Doug's idol Ross "The Boss" Rhea (Liev Schreiber) is demoted to the minors after serving a 20-game suspension for slashing an opponent in the head from behind. Three years prior, Rhea hit and concussed the highly-skilled prospect Xavier Laflamme (Marc-AndrÃ© Grondin), who has had trouble recovering from the incident due to the fear of being hit, resulting in his being stuck in the minors. After earning the nickname "The Thug," Doug was hired by a farm team in Canada where Laflamme plays, the Halifax Highlanders, to protect Laflamme and be his roommate.
The Highlanders experience success with Doug as their enforcer, and he quickly becomes popular among fans and teammates, much to the chagrin of his parents and Laflamme, who loses ice time and the alternate-captaincy to Doug. Doug becomes romantically involved with Eva (Alison Pill), a hockey fan with a penchant for players.
With four games left on their schedule, the Highlanders need two wins to secure a playoff spot. On a road game in Quebec, after an opposing player concusses Laflamme with a heavy hit, Doug savagely beats the player unconscious and is suspended for the next game against Rhea and the St. John's Shamrocks. Doug encounters Rhea at a diner, where Rhea dismisses Doug's claim that he is a hockey player, calling him a goon. Though Rhea acknowledges Doug's physical prowess and gives Doug his respect, Rhea warns him that if they ever meet on the ice, he will "lay [him] the fuck out." The Highlanders, with Doug suspended and Laflamme hospitalized, lose to the Shamrocks.
Doug reaches out to Laflamme, and promises him he will always have his back on the ice. In their next game, the Highlanders lead 1â0 thanks to strong teamwork between Doug and Laflamme. In the final seconds, Doug blocks a slapshot with his face and his ankle is injured in the ensuing scramble. The Highlanders win, but need a win against Rhea and the Shamrocks in their last game for a playoff spot.
After two periods, the Shamrocks are beating the Highlanders 2â0. Rhea and Doug drop the gloves in the third period. Doug is knocked down first, but Rhea calls off the linesmen and allows him to get back up. Doug manages to break Rhea's nose, but breaks his previously injured ankle in the process. Doug manages to stand back up and knocks out Rhea with a vicious left hook. Eva and his teammates help a seriously injured Doug off the ice and Laflamme, inspired by Doug's efforts and Rhea's defeat, scores a natural hat trick, giving the Highlanders a 3â2 lead. As the game enters its final minute, the final scene has Eva comforting Doug in the locker room as he comments, "I think I nailed him."

Question: How does Doug manage to knock out Rhea?
Answer: Vicious left hook.