For Chelsea , that's how you build a connection, and a team. As for West Ham United , that's how you get into another tailspin, but it's still hard to say how they threw that away.
It was probably the substitutions of both managers.
Whatever you put it down to, Chelsea went from getting booed off at half-time when 2-0 down to scoring a vintage stoppage-time winner for a 3-2 victory. The melee at the end, which resulted in a Jean-Clair Todibo red card, only fed into the raucous feeling.
It is actually the first occasion in Premier League history when the five-time Premier League champions have come back from two down at half time. A different legacy is building.
Jarrod Bowen opened the scoring with a cross that beat Robert Sanchez and ended up in the net (Reuters)
It is not, however, the first time that Liam Rosenior has introduced real change to turn a game in the second half. This victory was actually the second time this week, after the turnaround in Napoli on Wednesday. And another 3-2 at that.
If that had the symbolic significance of eliminating a former Chelsea title winner in Antonio Conte, while securing automatic qualification in the Champions League, this had the pure joy of beating hated local rivals.
"There's a resilience and a spirit and a fight," Rosenior beamed.
West Ham fans were left to protest their ownership. They might find common cause with Chelsea supporters there in another setting, where questions about their own ownership have fed into a certain reticence about Rosenior.
A feeling has pervaded that it wouldn't take too much for the young coach to endure hostility, a situation that was brewing at half time.
Crysencio Summerville blasted in West Ham's second goal (Reuters)
While there might be questions about his media delivery, about his youth, about his experience, about the club priorities in his appointment, he is offering compelling arguments about his coaching nous. That is really the only place Rosenior can prove himself, and he is doing it.
The new Chelsea coach turned this around through pure tactical acumen, and a quality that shows why he was picked for this specific role with all its parameters.
Sure, he might have got his starting line-up wrong for the second game in a row, having also made seven changes from Napoli. Alejandro Garnacho also had one of his most ineffective displays since joining Chelsea. Some of that may be fatigue, from the monotony of so many midweek games, which is an issue for all of the European qualifiers.
Rosenior admitted as much after the game, pointing to how it led to poor decision-making in the first half, from so few training sessions. He described a "lethargy".
"You need a squad."
He used it.
Through that, Rosenior also showed the assertiveness to rectify the first-half problems. Garnacho was hauled off, an erratic Cole Palmer was moved right, and the livewire Joao Pedro was introduced on the left.
The Brazilian electrically charged a second successive match. He already has the look of someone who can properly dominate a campaign for Chelsea. If he was more consistently fit, we might be talking about one of the players of the season.
It was Joao Pedro's header here, after quick thinking by Wesley Fofana , that started to turn the match. West Ham previously looked like they had Chelsea where they wanted them.
With Jarrod Bowen having scored a somewhat freakish cross to make it 1-0, Nuno Espirito Santo had one of his ideal situations, where his side could just break.
Joao Pedro came off the bench to score and try and change the momentum in the match (Reuters)
Chelsea only seemed to be playing into that, as they kept getting caught in a trench just outside the West Ham box. Crucially, Garnacho was giving so little trouble that Aaron Wan-Bissaka could surge forward, further freeing Bowen. It was from that that West Ham had a second, with Crysencio Summerville firing in superbly.
Chelsea needed something to change.
Rosenior altered the attacking line-up, the shape and the emphasis. It wasn't too long until West Ham were penned in. There was then a crucial switch from West Ham. Nuno, as if unable to escape his own nature, brought on Max Kilman .
"We tried to survive," Nuno said in his post-game press conference, before leaving somewhat abruptly. That applies to more than just this game.
Marc Cucurella's diving header brought Chelsea the equaliser (Getty Images)
Rather than strengthen the backline, though, the decision only had the vintage effect of putting it under further pressure. Likewise, it ensured West Ham went from really looking like they were on a survival push to looking down the pyramid again.
Nuno denied he had gone overly defensive too early, saying he picked what he felt fit. They had ceded all space, though. Chelsea were now filling it with a lot of energy and intensity.
A comeback never seemed likelier. Chelsea even got the equaliser relatively early, as Malo Gusto fired over from deep. Liam Delap was actually denied by Kilman, but he was so withdrawn that he headed it onto his own bar, allowing Marc Cucurella to dive in for a header.
At that point, at 2-2, the energy had transformed so much that you would have expected a 5-2 victory.
Chelsea arguably got the type of victory that was even better: a stoppage-time winner. There was Enzo Fernandez to seal it, in the 92nd minute.
Enzo Fernandez celebrates his stoppage-time winner for Chelsea against West Ham (PA Wire)
The late melee only played into the mayhem.
In contrast to half time, where he admitted he would have booed himself, Rosenior said he could feel the crowd were with them two minutes into the second half.
He now has a fifth win in a row in all competitions, six from seven, and another sign of his pure coaching nous. He also has an important big joyous moment.
