Four overs to go, 43 to get, six wickets in hand. These are situations that usually favour the batting team, but things are a little different when Jasprit Bumrah has the ball.

The first ball of the 17th over, Bumrah's third, is a case in point. It's the ball that has troubled New Zealand throughout the series - the wide, dipping slower ball that batsmen seem to pick up a fraction late from his whirling-limbs action. Tim Seifert looked to hit one of these over the covers in the first T20I, and ended up dragging a catch to long-on, reaching for the ball and losing his shape completely.

Kane Williamson is on strike, on 71, and even he doesn't pick it up properly. He goes for a big off-side whack, and fails to connect.

The next ball is another slower one, but fuller. Williamson doesn't really time his lofted drive. He slices it closer to deep cover point than he perhaps intends, and if he'd hit it a few inches higher, a few inches closer to the fielder, who dives desperately to his right… well, he hits it where he hits it and it streaks away for a lucky boundary.

Two balls, four runs, moral victory to the bowler, but those don't count for anything. New Zealand now need 39 from 22.

Williamson's innings has already contained a number of brilliant hits. In the 16th over, he had lofted Shardul Thakur for an effortless six over long-off, inside-out, after skating away from the line of the ball. If that ball, landing right in his hitting arc, was a bit of a gift in terms of its length, Williamson's two previous sixes had required a lot more work on his part.

These came off Ravindra Jadeja in the 15th over. First, he got a wide-ish delivery outside off stump, on a more-or-less typical Jadeja length and at a more-or-less typical Jadeja pace. Williamson hit it back over the bowler's head for six. The length wasn't full enough for the regulation lofted drive, so he ended up playing something like a straight slog-sweep. He stretched his front foot an unusually long way forward, and planted it to the leg side of the ball, to ensure he was close to its pitch while still having enough room to swing his bat through a powerful arc.

Getting near the pitch of the ball was vitally important on this Seddon Park surface, where the ball was gripping and turning considerably. Jadeja, when he bowled that ball, must have been pretty confident Williamson wouldn't be able to.

With that one massive stride, Williamson did two things. He hit that ball for six, and he messed with Jadeja's mind. If a batsman can get that far forward - Williamson had done it earlier too, to slog-sweep Yuzvendra Chahal, so this wasn't a one-off - a bowler's natural response is to look to bowl a little shorter, which is what Jadeja did. It wasn't a genuinely short ball - it was back of a length, and sliding into the stumps with the angle, but Williamson expected it, rocked back in a flash, and pulled it for six more.

Thanks to shots of that nature, Williamson was scoring at an absurd rate on a pitch that wasn't straightforward to bat on. He scored 26 off 15 balls when Mitchell Santner made 9 off 11 at the other end. He clattered 44 off 20 when Colin de Grandhomme plodded to 5 off 12.

Those two Bumrah deliveries, then, infuse proceedings with a bit of tension. What's he going to do next? How will Williamson respond?

The third ball of Bumrah's over is one of the defining balls of the match. Williamson takes a back-and-across step just as Bumrah delivers it, so his back foot is outside the line of off stump at release. The ball is on a length, at full pace, angled in but directed at a fourth- or fifth-stump line, and Williamson, with his step across the stumps, is able to hoick it over the leg side, into the big gap between deep backward square leg and deep midwicket.

On TV, Mike Hesson, the former New Zealand coach, suggests this step across the stumps is pre-planned. New Zealand, he surmises, have sussed out that Bumrah has bowled most of his full-pace balls at the stumps or just outside off, and his slower balls on a wider line. By getting closer to the line, Williamson should be able to hit the slower balls with more power, and use Bumrah's natural angle to hit the quicker ones into the leg side.

At his post-match press conference, Williamson denies - or refuses to disclose - that any such plans were in place. "I don't know," he says. "Maybe ask like AB [de Villiers] or some of these guys that are superstars that might have some bits of gold for you. For me, it was trying to look for areas in the moment on that surface."

The leg-side boundary, from the end Bumrah is bowling, is significantly shorter than the opposite square boundary, so that may well have been the simpler explanation.

Either way, Williamson's movements have got into Bumrah's head now. In what might be (remember, this is all conjecture) an effort to surprise Williamson and serve up something he may not have planned for, he goes for the wider line again, but at pace. Maybe he's looking for the wide yorker. If that's the case, he misses his length, and doesn't bowl it full enough, and Williamson hits it over cover. Four more.

Williamson has hit the previously un-hittable Bumrah for 4, 4, 4, and New Zealand need 31 from 20.

The feeling that New Zealand have a plan against Bumrah resurfaces in his next over, when Ross Taylor mimics Williamson's movement across his stumps, picks up a length ball from just outside off stump, and wallops it to the square leg boundary.

Then Williamson gets on strike again, and plays what could well be the shot of his innings. Bumrah has sent fine leg back, and this perhaps tells Williamson that his line is going to be straighter. He shuffles across anyway, and meets a middle-stump yorker with an impudent flick. If he misses, he's bowled. He middles it, and the fielder at fine leg, sprinting desperately and flinging himself to his right, can't cut off the boundary.

New Zealand need 11 off 8 now, and it becomes 9 off 6, and 3 off 5, and 2 off 4, before… You know what happens next, but that is immaterial, or should be, when you evaluate Williamson's innings. He's scored 95 off 48 balls and, in his time at the crease, the batsmen at the other end, plus extras, have made 36 off 35. When he's dismissed, he leaves New Zealand two runs to get off three balls. Whichever way the result may have eventually gone, that's the definition of a match-winning innings.

Karthik Krishnaswamy is a senior sub-editor at ESPNcricinfo