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It's all in the geometry - how Shami and Siraj put the skids under Australia

There are very good reasons for Indian quicks doing better in India than visiting quicks; it's got to do with angles and trajectories and much else

Mohammed Shami has seven wickets at an average of 14.42, from two Tests, taking a wicket every 26 balls.  •  BCCI

Mohammed Shami has seven wickets at an average of 14.42, from two Tests, taking a wicket every 26 balls.  •  BCCI

David Warner came to India with a point to prove. His previous two tours of the country had brought him 388 runs in eight Tests, at an average of 24.25. There were question marks over his game against spin, particularly against his old nemesis R Ashwin.
Warner's tour has ended, prematurely, and his average in India has dropped further, to 21.78, but while he has struggled against spin, he has only been out to it once in three innings. The larger share of the wounds he is carrying back to Australia - a concussion, a fractured elbow and two dismissals - have been inflicted by India's fast bowlers.
Mohammed Shami has dismissed Warner twice, both times in a clinical manner that has homed in on an old weakness - a lack of footwork, and consequently a tendency for his bat to come down at an angle while defending good-length balls angling into him from right-arm around.
Both times, Shami homed in on a top-of-off-stump sort of line and length. Both times, he pinned Warner to the crease. In Nagpur, the ball kept going with the angle into Warner, more or less, and bowled him off the thigh pad, beating the inside edge of his bat as it sliced down from gully to mid-on. In Delhi, the ball straightened off the deck, and kissed the outside edge as Warner's bat sliced across the line in similar manner.
Two classic Shami dismissals, in a classic Shami series. Over the first two Tests, he's taken seven wickets at an average of 14.42, bowling only 30 overs but taking a wicket every 26 balls.
Mohammed Siraj hasn't taken a wicket since getting Usman Khawaja lbw with his first ball of the series, but he's only bowled 18 overs, and has looked extremely awkward to face at times, particularly when he tormented Warner with the short ball in Delhi.
As a combination, India's quicks have averaged 20.12 over this Border-Gavaskar series. Pat Cummins and Scott Boland, meanwhile, have combined to average 51.00.
Two Tests is a small sample size, of course, and batters have achieved fairly similar control percentages against Shami (79.0), Siraj (80.6), Cummins (80.6) and Boland (81.4), suggesting that there may be a degree of randomness to the skewed averages.
But the skewed averages have been par for course in nearly every home series India have played over the last decade. It's one thing to induce uncertainty, and another to translate uncertainty into wicket-taking opportunities.
In five of India's last seven home series, their fast bowlers have collectively averaged below 21. In each of those five series, the opposition quicks have averaged over 35.
How they have done it is partly down to home advantage, and of bowling in a style that heightens their threat on lower-bounce pitches. Shami (20.63), Siraj (22.85), Jasprit Bumrah (15.64) and Umesh Yadav (24.71) all average below 25 at home since the start of 2013. Skiddiness, in one way or another, defines all of them.
What exactly do we mean when we call a fast bowler skiddy? There's more to it, but at the simplest level it's all about geometry. Shami, by definition, releases the ball at a significantly shallower angle than the 6'5" Cummins does to hit the same spot on the pitch. The ball comes off the pitch at a shallower angle too, which means Shami can threaten the stumps from a wider range of lengths than Cummins. It's why batters are so often rooted to the crease by Shami deliveries that uproot their stumps.
In South Africa last year, the uniformly skiddy nature of India's pace attack became a disadvantage on pitches where the ball that climbed from a length was the biggest threat to batters. Catches at gully and short leg were likelier occurrences than bowled and lbw, and South Africa's quicks put their considerable height advantage to telling use to engineer Test wins in Johannesburg and Cape Town.
Tall, hit-the-deck quicks tend to prosper in bouncier conditions, and Australia and South Africa have always had an abundant supply of that sort of bowler. India haven't always had an assembly line churning out world-class skiddy fast bowlers, but they have had one over the last decade.
India's skiddiness advantage is most apparent in how well their fast bowlers attack the stumps: 139 of their 253 wickets in home Tests since the start of 2013 have been bowled or lbw. That's nearly 55%. Visiting teams' fast bowlers have taken 237 wickets, out of which only 93 - or 39% - have been bowled or lbw.
But skiddiness is also awkward to face from shorter lengths. The short ball comes off the pitch quicker and at a lower height than expected, and tends to cramp the batter for room. When Siraj hit Warner on the elbow and the head in Delhi, it was evident that he had tried to play the pull both times with his elbows tucked in rather than with a full extension of the arms.
Putting batters in these sorts of positions can create genuine wicket-taking chances. Since the start of 2013, according to ESPNcricinfo's data, India's fast bowlers have taken 75 wickets from short and short-of-good-length balls in home Tests, at an average of 22.49. In the same time, visiting quicks have taken 71 wickets from these lengths while averaging 40.73.
In Indian conditions, fast bowlers usually only operate in short bursts, whether the ball is new or old. This can be a mixed blessing. You can bowl flat-out, knowing that the spinners will be back soon to take over the workload, but you also know you have a limited window to make an impact in. It takes an incredible amount of skill and intelligence to create chances over these short bursts. Zaheer Khan did it frequently in his pomp, and over the last decade, his successors have taken it to a new level as a collective.
For the visiting batter, then, there's no respite. If Ashwin, Jadeja and Axar don't get you, Shami and Siraj probably will.

Karthik Krishnaswamy is a senior sub-editor at ESPNcricinfo