A curious thing happened during the recent Australia-Pakistan ODI in Brisbane. In the fifth over, Mohammad Amir bowled a peach of a ball to David Warner. With the seam canted towards slip, the delivery started on the line of middle stump before swinging away, evading the left-hand batsman's attempted flick to leg by some distance, before clipping the bails. It was an exquisite, almost unplayable outswinger, behaving much as the bowler intended, and defeating a highly capable batsman in prime form.

It is the delivery that followed, however, that is more interesting, at least for the purposes of this article.

This time it was Steven Smith taking strike and facing the first ball. The ball left Amir's grip with the seam pointed in the same direction as the previous delivery, apparently intended to move in to the right-hander. It swung away instead, and Smith, walking into an off drive, edged a catch through to the wicketkeeper.

"What a delivery," remarked one Channel Nine pundit. "Totally different to what he bowled to Warner. This was going away from the right-hander, Smith… he was expecting the ball to come in." Amir had gone past Warner's bat on a few occasions with deliveries swinging away. Smith might therefore have expected the ball that got him to have behaved similarly.

In Barbados in May 2015, right-arm fast bowler Jerome Taylor was bowling the fifth over of England's second innings to Jonathan Trott. The fifth delivery was full, swung in slightly, went past the inside edge and struck the right-hander on his pads. Billy Bowden's crooked index finger shot up in response to an almighty appeal and the batsman trudged off after being dissuaded by his partner and captain, Alastair Cook, from requesting a review. Slow-motion replays clarified that though the delivery swung into the batsman, the seam was pointed in the direction of slip, hinting that the delivery ought to have swung away. Previously, Taylor had been largely swinging the ball away from Trott, so the batsman was no doubt surprised to see this one swing in.

"Whenever the old ball swings, it is immediately said to be reversing. Contrast swing is rarely mentioned, and yet, that is more likely the correct characterisation"

The television commentators didn't seem to be able to explain what had happened either. In the end they appeared to put it down to being just one of those things that sometimes occurred. Something similar, perhaps, to one spinning delivery turning and another going straight on - just some kind of natural variation.

But there is much that science can explain, such as the effects of the aerodynamic forces acting on a cricket ball flying through the air. For conventional swing, the ball is expected to deviate in the direction in which the seam is pointed, while for real or true reverse swing, the ball swings in a direction that is opposed to that of the seam.

Since the two deliveries under scrutiny (Taylor to Trott and Amir to Smith) unmistakably moved in opposition to the orientation of the seam, both are probably best explained by real or true reverse swing.

Now, many of us have been led to believe that the new ball does not meet the requirements necessary for reverse swing to transpire. To enable the technique, it is widely believed that the ball has to have aged at least 40 overs, with one side tended to and polished (without any external substances, of course) and the other allowed to grow rough and unkempt. That is indeed one method of facilitating it, but studies have shown that reverse swing can be, and is, achieved with the new ball as well.

One requirement for it to occur is that the bowling speed needs to be around 90mph or higher. Amir's deliveries that sent back Warner and Smith had similar seam orientation. Both were delivered at or very close to 90mph. So why did they bend in opposite directions? It is not widely known, but even with a brand new ball, there is often a subtle but noticeable difference in surface roughness between the two sides (due to differences in embossments or markings). There is a possibility also that one of the sides picked up a slight blemish even after only four overs.

The position of the seam corresponds to the direction of the swing - when the swing is conventional Getty Images

One hypothesis is that in Warner's case the ball was probably released with the relatively smooth side facing the batsman, and in Smith's case, the ball was flipped over so that the relatively rough side was facing him. It is unlikely the bowler did this deliberately. One of the peculiarities of reverse swing is that the bowler need not alter his grip on the ball or his action. Therefore, the right-arm fast bowler who normally swings the new ball away from the right-hand batsman will often swing the old ball in without any change in bowling technique. Fidel Edwards, for instance, typically swings the new ball away, but is known to deliver searing inswingers with the old ball. Dale Steyn also mostly bowls outswing with the new ball and inswing with the old.

Often confused with reverse swing is what is properly known as contrast swing. How is it different from reverse? Contrast swing occurs when the ball is released with the seam straight up, rather than angled towards slip or leg slip. The ball will swing when there is a contrast in surface conditions between the two sides of the ball. The greater the disparity in roughness and smoothness, the greater the inclination for the ball to change direction. At the bowling speeds normally achieved by fast bowlers, say between 70 and 85mph, the ball will swing towards the smooth or shiny side.

Indian fast bowler Mohammed Shami consistently bowls with the seam straight up (contrast mode), and normally operates at speeds of up to 85mph (close to 140kph). ESPNcricinfo's match report from Shami's Test debut, against West Indies in 2013, says: "Shami bowled consistently in the late 130s on a slow pitch, and was a different proposition with the ball scuffed up, finding movement that had not been there for him with the new one."

The movement was noticeable only when Shami returned to the attack in the 42nd over. Unsurprisingly, it was referred to as reverse rather than contrast swing at the time.

So are there any advantages of contrast swing over reverse swing?

One, it is much easier for non-swing bowlers to release the ball with the seam straight up rather than angled. Contrast swing is also possible with the seam completely bashed in - a common occurrence in the subcontinent. A prominent seam plays a critical role in conventional and reverse swing. Whenever the old ball swings, it is immediately said to be reversing. Contrast swing is rarely mentioned, and yet, that is more likely the correct characterisation. What we have seen, more often than not, is that both contrast and reverse swing are lumped together and labelled reverse.

Can a brand new ball achieve contrast swing? If both sides of a new ball are in similar condition, it follows that there will be no contrast swing.

But, as we pointed out above for the reverse cases, if there is a slight difference due to different embossments, or one side picks up a blemish, even a slight one, then there is a chance for contrast swing. And since the difference in roughness will be relatively small, it will only occur at the higher speeds, say around 90mph or higher.

It is possible to identify the type of swing a bowler is producing by making note of the positioning of the seam and the direction of the swing. If they are coincident, then it is conventional swing; if opposed, it is reverse swing; and if the seam is pointing straight down the pitch, then rest assured that you have just observed contrast swing.

Swing is one of the fast bowler's most important weapons. Increased knowledge of its workings and scientific underpinnings should not only lead to a better understanding of the skills on display but also enhance our appreciation of the nuances of the great game.