It is the 43rd over. Bangladesh's finisher is at the crease. Mahmudullah has been biding his time. He's only 20 off 40 but he's now itching to go.

Mitchell Santner comes on. A flag bearer for fingerspin in limited-overs cricket. He sees the batsman making room and, in the split second between load up and release, decides to slow his pace down and shift his line well outside off.

Mahmudullah ends up having to reach for the ball - losing his balance and, as a result, his power - and spoons it to cover. Kane Williamson is there, taking a catch so simple it makes the cricket look silly.

The New Zealand captain rarely shows any emotion on the field. In the 2015 World Cup, he hit Pat Cummins for a straight six to conjure a one-wicket win. Forty thousand people were going mad at Eden Park. But the most he could muster was one fist pump. In slow motion. Here, he looks Santner square in the eye and starts clapping.

Later that same night, our hero unfurls the picture perfect cover drive and seals a victory almost as nerve-wracking as Auckland four years ago. He then starts to walk back to the dressing room but feels a little push on his back. It's his batting partner Lockie Ferguson basically saying without saying, "What are you doing, you idiot. Get in that spotlight and lead us off the park."

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The world may underestimate Santner. He's No. 26 on the ODI bowler's rankings. But New Zealand don't. He's the first spinner on their team sheet. He gives them ten overs most times and, somehow, despite small grounds and flat pitches, finds ways to contain batsmen.

It's his one job and it's crucial because it makes up for a weakness.

New Zealand's batting - especially their finishing - is wobbly. They can't always make and chase 350. So they rely on the bowlers to set them up with softer targets. They are like Pakistan in that way. That's why there are three slips when Trent Boult runs in with that glossy new ball. That's why Santner's economy rate - 4.87 - is valued above his strike rate - 42.2. He creates pressure at one end so his mates can strike from the other.

A lot of things come together to make Santner produce such numbers. His instinct - telling him when to toss the ball up and when to slide it through; his use of angles - going around the wicket to left-handers so even though the ball turns into them, the angle would test the outside edge; his bravery - he usually takes the pace off when people are trying to hit him for six.

Simon Doull, on commentary, highlighted that point in the game against Bangladesh. And the reason it is important is because modern-day batsmen are used to spin bowlers going quicker and flatter when under pressure. So they set up expecting Santner to do the same. But he seems to be aware that by slowing it down, he can make them lose their shape and mistime their shots. The stats bear that out too. Only Mohammad Nabi (700) and Rashid Khan (714) have conceded fewer runs in boundaries than he (884) has.

Now, full disclosure. That list comes with a pretty big rider - minimum 50 innings bowled since June 9, 2015 - and is therefore only 14 strong. But that too is a sign. It shows that, from the day he made his debut, Santner has been among the most trusted players in 50-over cricket. After all, no captain throws the ball that many times to someone unless they know they will deliver.

Let that sink in for a bit. New Zealand have placed utmost belief in fingerspin even though all of us have moved on to wristspin. It's like going to an ice-cream shop and ordering double chocolate fudge despite there being a sale on the hipster flavours. And it's worked. Take a look at the batsmen Santner's gone up against, and kept quiet.

That's two GOATs... Enough hard-hitters from the two countries tipped to win this World Cup... Hell, he's even kept a lid on the very best left-handers. With all that in mind, is it any wonder that the original left-arm-spinning Black Cap Boy Wonder is raving?

"Mitch Santner was the most impressive," Daniel Vettori wrote in his ICC column after the Bangladesh game. "It shows how much he has come on that New Zealand felt they could use him late on, in overs 40-50, he has so much control now and is both a defensive and attacking spin bowler. He went for one four and one six in the whole innings and was a really key part of that attack.

"Mitch has been exceptionally unfortunate with injuries but now we're seeing exactly why the selectors picked him from a young age. They saw the talent and he's shown he has the ability, not just with the ball but in the field and with the bat too."

It's remarkable that a 27-year-old slow left-armer, from a country that doesn't have a spin culture, in a world thirsty for X-factor bowlers, has become so good. And the funny thing is, this is only the beginning.

Alagappan Muthu is a sub-editor at ESPNcricinfo