Genji is a supremely useful offensive hero, but he often lacks the powerhouse throughput damage of other offense heroes such as Tracer or Soldier: 76. That problem melts away when Genji pulls out the Dragonblade, which allows Genji to cut through entire teams in just a few swings of the sword.
In the video, Fainted00 uses their Dragonblade, but has it canceled out by the enemy Zenyatta's Transcendence. Their team stays in the fight, however, and when the Dragonblade runs out the enemy team blasts Fainted00 with all they've got. Fainted00 gets the Deflect up in time, however, translating all that damage into ultimate charge.
In just an instant, Fainted00 has their ultimate again and pulls a second Dragonblade. This one proves stronger than the first, letting Fainted00 cut the enemy team to ribbons and win the team fight.
Photo courtesy of Blizzard