Sometimes, you get a ball that's just too good. Sometimes, you get two in a Test match.

Virat Kohli got two devilishly good balls in Christchurch - one from Tim Southee, and one from Colin de Grandhomme. Both threatened to swing away from him, and both nipped back off the pitch. Both were pitched on the fuller side of a good length, bringing the batsman forward but still leaving him far enough from the pitch of the ball for the seam movement to do maximum damage.

Maximum damage is exactly what took place. Kohli looked to drive Southee down the ground in the first innings and to defend de Grandhomme in the second, and on both occasions, the ball went past his inside edge and hit his front pad, bang in line with the stumps.

Both balls were starting to curve away from the batsman until they pitched and abruptly changed direction. It's the physics of backspin - inswing bowlers usually tend to bowl the legcutter as a variation, and outswing bowlers the offcutter - but sometimes it's not even in the bowler's control when it happens.

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"I think that's a question for Tim or for Colin," Trent Boult said in his press conference at the end of the second day's play, when was asked whether they meant for the ball to nip back in like it did. "But I think they call that natural variation, pretty much, it's just one of those things.

"You're trying to bowl a certain ball, and you slightly get a bit of variation with whatever, but yeah, the bowling group as a whole, I think the approach is to not give them too many soft deliveries, group the balls nicely, and then almost let that natural variation take care of itself. Yeah, we're lucky to get two that lined up and hit him on the pads nicely, and yeah, it was a good feeling to see the back of him."

Sometimes you get a ball that's just too good, and sometimes it's not even wholly intentional on the bowler's part. But could Kohli have played them better?

Replays of both dismissals suggest Kohli could have.

Against Southee, his head was falling over to the off side, ending up on a roughly fifth-stump line for a ball that pitched a fraction outside off stump and struck him in front of middle and off. He ended up slicing across the line of the ball in an attempt to drive down the ground, with his bat a long way in front of his body. Playing with a straighter bat, and closer to his body, might have allowed Kohli to survive.

Kohli's head was in a much better position against de Grandhomme, over the line that the ball may have ended up in had it followed its initial outward trajectory. He was also playing a lot closer to his body, so there was more reason for him to feel he simply got an unplayable delivery.

But on both occasions, Kohli did something he almost never does when he's batting fluently and scoring lots of runs, a little thing that was more easily observable from side-on. Usually, Kohli's head is on top of the ball, roughly above his front foot. It allows him to play on-drives or flicks through midwicket even when he looks a little off-balance from front-on. It allows him to cover drive safely even when he's not necessarily got his front foot close to the pitch of the ball.

In Christchurch, his weight wasn't going forward, and his head was ending up behind his front foot, forcing him to play around his front pad. And if your head isn't getting on top of the ball, there's one other consequence: you end up playing your drives, whether through on or off, away from your body, with your weight not fully forward.

Think back to Kohli's first-innings dismissal in the first Test in Wellington. The cover drive he attempted off Kyle Jamieson there was perhaps not on, given the ball's line and length - it was closer to a wide length ball than a wide half-volley - and also India's match situation. But as risky as it seems, he often drives similar balls for four. What was different, again, was his head position, well behind his front foot, forcing him to throw his hands at the ball.

If Kohli's head position has indeed been the issue dogging him right through this Test series, and perhaps the whole tour of New Zealand, what could have caused it? It's hard to say, but one thing leaps out as a possible reason. Right through this tour, New Zealand's use of the short ball has been spoken about and analysed endlessly, and there's a reason for it: they bowl a lot of short balls, Neil Wagner in particular. India must have planned extensively for it as well; as early as the eve of their three-day warm-up match in Hamilton, a player as inexperienced as Shubman Gill mentioned in a press conference that India would need to be wary of giving New Zealand too many wickets off the short ball.

When you have the short ball in the back of your mind, it can't be easy to get your weight moving forward when the ball is full. Perhaps this is what has held Kohli back, and caused him to endure one of his worst-ever series.