Momentum (not that kind) and positivity (not that kind, either) explain England's sparkling victory in the first Ashes Test.

Momentum and positivity are typically misleading clichés. Momentum usually means the assumption that what has just happened is likely to influence what is just about to happen. Not so. Sport is much more interesting and surprising than that - just ask the professional gamblers who constantly struggle in pursuit of predicting the future.

Positivity is usually framed as taking risks, having a go and releasing the shackles. But, again, there is far more to it than that. Positivity is more about mood and character than shot selection.

So my thesis is that there is a far deeper kind of momentum and a much more complex dimension of positivity. The first Test showed how important they are.

The swing in momentum, away from Australia and towards England, did not begin in Cardiff. The contrasting performances there were a symptom of that momentum shift rather than the cause.

England looked young and peppy, full of hopefulness and energy. Australia looked like they were hanging on - to their places in the team, to their status in world cricket, perhaps to their sense of purpose and conviction.

Two moments hold as apt symbols for the contrasting moods of the teams. Mark Wood, having found the outside edge of Adam Voges' bat, leapt into the air with boyish delight, grinning uncontrollably. The wicket was the prompt, of course. But the underlying source of joy was his expressive competitiveness. He was having fun out there and he couldn't hide it. The look on Wood's face was almost as significant as the wicket itself, as though his personality couldn't be kept under wraps.

England looked young and peppy, full of hopefulness and energy. Australia looked like they were hanging on - to their places in the team, to their status in world cricket, perhaps to their sense of purpose and conviction

Shane Watson, on the other hand, who was preferred to the in-form Mitchell Marsh on the strength of his bowling, ambled through 13 wicketless overs. He bowled tidily enough, as you would expect. Each and every walk back to his mark, however, looked like a psychological as well as physical struggle, as though Watson was fearful of getting injured merely reaching the top of his mark, let alone running in to the crease. He has always been a cautious, injury-prone bowler. But his habit of conveying anxious self-preservation seemed worryingly in tune with the rest of his team.

Wood: vitality, joy, athleticism, expressiveness.

Watson: hanging on, doing a job, trying to survive undamaged.

Let me clarify at the outset. Australia can count on some wonderful cricketers. Dismiss them out at your peril. With four matches still to play, there is plenty of time for a comeback.

In the broadest sense of the term, however, the momentum surely favours England. That momentum may not be fully expressed in this series; Australia may preserve the old order a little longer. But ask yourself about the general trajectory of the following Test careers: Jos Buttler, Ben Stokes, Mark Wood, Joe Root. All are coming men, relishing the process of discovering the scope and potential of their talent. They are beginning to realise how good they might be, and it is proving to be a very pleasant learning curve.

In contrast, occupying similar roles for Australia, what is the direction of travel for Brad Haddin, Shane Watson, Mitchell Johnson and Michael Clarke? All are fine players, all may yet turn the series towards Australia. But they are summoning all their physical and mental reserves for one extra push, dipping into the bucket of their psychological reserves hoping to find a little something left at the bottom. Maybe there is something still to give, maybe there isn't. More certain is that the end is in sight.

Forget who won the last session or the latest skirmish. More fundamentally, momentum is a question of the team's collective DNA. Is this team growing and developing, excited about the future? Or is it digging its fingers into the rock face, trying not to slip down from the summit?

Which brings us to "positivity". The most important aspect of positivity is not team talks and tactics but selection. Cricketers under pressure tend to revert to what they know. Coaches can only mould them to a degree. Who they are underneath tends to come out eventually.

What, then, are the factors that have led England's cricket team to play with such positivity? A few days in Spain getting to know the new coach, Trevor Bayliss, might have provided a useful nudge. Much more central is the make-up of the team, the personalities out on the field. England's new-found optimism doesn't derive from team talks or flip charts but from individual confidence and hopefulness. The team's emerging young group - the future core of the side - are enjoying growing into their careers simultaneously.

The mood of the first Test was set by young England players, but the beneficiaries included the old hands. It is hard to remember a better spell of new-ball bowling by Stuart Broad and James Anderson than the first session of the fourth morning. They were relentless, controlled, hostile and skilful. It was sophisticated, high-class Test cricket.

Ian Bell, who is prone to introspection and underplaying his hand, got into the act too, with a sprightly and timely 60 in the second innings. Broad, Anderson and Bell seemed to feed off the infectious enthusiasm. They too were suddenly having fun. After all, it is far more enjoyable to be an experienced player in a healthy side full of emerging talents.

"We are going to be positive, play without fear and express ourselves." Those often repeated words are easy to say, very hard to do. Except, that is, when players cannot do otherwise, when self-expression and optimism are not a tactic but the unavoidable reality of their character. England are playing a positive brand of cricket because they've selected a positive bunch of cricketers.

The most underrated team qualities are optimism and innocence. England lead Australia in that match-up. That's why they have the momentum.

Ed Smith's latest book is Luck - A Fresh Look at Fortune. @edsmithwriter