It starts out with the two Roadhogs engaging each other in melee completely off point. Bamboozle, the red team player lets themself get a little too close to the edge and the blue team player decides it's worth spending an ult just to get rid of one of the other team's tanks for a bit. They push Bamboozle off the cliff, but then makes the fatal mistake of turning their back before they're sure the red player is dead. Bamboozle decides that if they're taking a swim the enemy is coming with them and, while the blue Hog's back is turned, they manage a last-second grab with the chain hook, taking the other Hog with them into the sea.
A careful eye might notice that in the clip, Roadhog's chain might seem longer than it should, this is because the distance the hook can travel is based on the initial spot it was thrown from. So, even though Bamboozle's momentum is carrying them backward, the chain will still behave as if it was thrown from a stationary position. Keep that in mind if you ever get knocked off a cliff while an enemy player is still in range.