It was a complex offside situation. Sadio Mane had converted the rebound, seemingly giving Liverpool a 1-0 lead at the Emirates, but a linesman's flag had thwarted him. Mane had been offside when the initial pass was played, but wasn't interfering with play. When he was interfering with play, in the second phase, he'd been level with Roberto Firmino as the Brazilian volleyed towards goal. Mane was onside, the goal should have stood.
From the freeze-frames on the replay, we were looking at the position of Mane, the position of Firmino, and the position of the Arsenal defence. But not, crucially, at the nature of the pass.
It came from right-back Trent Alexander-Arnold. This wasn't a swinging cross around the defence and in behind the opposition, or a hopeful lofted ball into the channel. No, instead Alexander-Arnold had checked inside, dribbled towards the centre of the pitch, and then dinked a perfect ball over the top, with his weaker left foot, to put two Liverpool attackers running through on goal. It's the type of wrong-footed pass that even the Premier League's most gifted playmakers might not have attempted, played to absolute perfection, by a 20-year-old right-back.