I can’t argue with this at all:
What happened was Han shot another TIE fighter, one that wasn’t doing anything.
At that moment, Darth was still free to blast Luke from existence. And that’s when our nameless TIE fighter pilot comes in. He’s the one on the right up there.
At the sight of another ship getting blown up (in a battle where that has happened dozens of times already), he panics…
… and crashes into Darth Vader.
He literally shouts “Look Out!” before swerving directly into Vader’s fighter. Vader goes spinning off into space…
…the fighter pilot crashes into the trench, and Luke is free to fire his torpedoes and save the day.
None of that was due to anything the Rebels did right. Even if he thought he was in danger of being shot after losing a wingman, the TIE fighter pilot had room to fly in any direction. The trench was pretty wide, and he had the entire infinity of outer space above him. Instead, the direction he chose was “directly into the boss.” We’re going to assume his family did not get his pension–the Empire is probably pretty strict about panicky idiots winning the war for the other team.
Incidentally, this also brings to light the fact that Han Solo had a once-in-a-lifetime shot to take down the Dark Lord of the Sith and squandered it on a wingman. Apparently stupid goes both ways, since if you have a three-man formation and the guy in the center has a completely different vehicle, odds are pretty good he isn’t just some grunt being escorted to the Imperial Birthday Party Wing of the Death Star while a battle rages around him. Han had the element of surprise and apparently his total pick of who he was going to shoot, because it was an instantly lethal shot.