If England win on Sunday, credit the players and Gareth Southgate. But also credit Owen Eastwood and whakapapa.

Eastwood is a softly spoken, modest, UK-based New Zealander of Māori descent, a lawyer turned performance coach who has been working with England’s footballers and whose book Belonging was presented to every member of the squad at the beginning of the Euros.

In some ways, Eastwood, who has also worked with the British Olympic Association, the Royal Ballet School and the command group of Nato is the secret weapon. On Thursday, in the aftermath of After Wednesday’s victory, he was in dialogue with Southgate.

Whakapapa (pronounce it farkapapa), he says, is the Māori way of explaining your place in the world, and your place in any tribe or family.

“Each of us is part of an unbreakable chain of people, back into our past to our first ancestors, and into the future, to the end of time. Everybody has their arms interlocked so it’s an unbreakable chain. The metaphor is that the sun first shone on our origin story and slowly moves down this chain of people, and when the sun shines on you that signifies your time.”

In a family, that signifies your life, in a sports team the time you get to wear the shirt. “When the sun is shining on you you have an obligation to make the tribe stronger, and ultimately what you achieve when the sun is shining on you is going to be your legacy, the memory that people will hold of you in the future.”

Five years ago, when Eastwood accepted the FA’s challenge to introduce identity and togetherness to the team, he immersed himself in the England story, in search of whakapapa. He studied the history, back to the first game against Scotland in 1872 when Cuthbert Ottaway was captain.

He spoke to past players, spent time with Jimmy Armfield, and Michael Owen, and found that sense of belonging and identity had been missing. Now they – the current squad – talk about it; belonging is important, and a part of belonging is what Eastwood calls a “story of us”.

Humans need to belong, it is an element of performance. “We try to signal to players that this is a place where you belong. You are respected, this is a safe place, we want you to be yourself and express yourself.”

But the England of Sterling and Kane, of Saka and Maguire is a very different one to Ottoway’s, surely? They are not trying to replicate anything from the past, Eastwood says. “We should be proud of our history, understand and respect it, but ultimately this is about us, a very diverse, young, technically different group of people. It’s about inspiration and a sense of belonging to motivate us to create our own story.”

That diversity is a massive strength, he says. “In that when the English people look at the team, they are seeing an expression of the diversification of the country. And you’re seeing them as people who respect and care about each other, they celebrate each other’s success, they are joyous being one. Gareth has spoken about that, this is somewhere for English people to look at what our identity as a country looks like and feel proud about it.”

Eastwood was born in New Zealand to a half-English, half-Māori father. Earlier this year he described how, after watching New Zealand’s all-conquering All Blacks rugby team prepare for games with their traditional haka, he wrote to the office of Ngāi Tahu, the Māori tribe from which his father was descended, asking for details about his heritage. He told Bookseller magazine the reply stated simply, “You belong,” adding: “It was like a person putting their big arms around me.”

Eastwood, who would later work as a lawyer to the All Blacks team, is too discreet to discuss individuals in the England setup, or go into detail about what is going on behind the scenes. He never refers to Southgate as “manager”. He talks instead about “leadership”.

An understanding of the environment and its impact on people is more important than charisma and passion. Plus clarity of thinking. “Humans flourish in environments where the leadership is consistent and composed; they don’t tend to thrive in environments where there are wild mood swings and inconsistency of behaviour.”

Creating an environment of trust and care is also really important in good leadership. “In order to trust somebody you need to know they care about you. If you know someone has your back, they might not always make a decision in your interest but you know that before they make a decision they will at least take your situation into consideration, because they care about you and there is a relationship between you. It makes you confident about your environment and more trusting in your leader.”

Listening to Eastwood, it is hard not to think about Jack Grealish not throwing a wobbly when substituted. Or hear Southgate always bigging up the players who have not made the team, the Ben Chilwells and Conor Coadys.

Throughout the tournament Southgate has talked about history and bigger journeys, about his “tribal elders” – Sterling and Henderson, the Harrys Maguire and Kane. A lot of what he has said and the way he has could have been torn from the pages of Eastwood’s book.

Eastwood lets slip for moment, gets personal about Southgate. “Yup, no question, I think he’s an outstanding leader. What happens on Sunday, I think people will regard Gareth as a very very talented leader.”

What about tomorrow then? Eastwood won’t be there; he’s never been to an England game, he doesn’t need to, he’ll watch at home in the Cotswolds with his family. Will they succeed though? “A successful team has a clarity about its higher purpose. The England football team has a purpose to inspire and unify the English nation. For me personally, I believe this team is successful already, because no matter what happens on Sunday, they’ve done that.”

Over to you then Raheem, and the Harrys, Bukayo, Jack, Kyle and the rest. And Gareth. The sun is shining on you, this is your time, go find your whakapapa.

Belonging: The Ancient Code of Togetherness by Owen Eastwood is published by Quercus, out now