Men like Trent Boult, Mitchell Starc, Mitchell Johnson, Wahab Riaz, Mohammad Irfan and Rahat Ali have made this edition of the World Cup truly spectacular. These bowlers, with their left-arm pace, have played pivotal parts in taking their teams through to the final eight. Left-arm fast bowlers have added a whole new dimension to this World Cup, which on the whole has been dominated more by bat than ball.
If there's a bowler who can make the batsman change tack before a ball is bowled, it has to be a left-arm fast bowler bowling over the stumps. When a right-hand batsman is facing a left-armer bowling over the wicket, he needs to open up his stance a bit, for otherwise the point the ball is delivered from creates a blind spot for the batsman.
A right-arm fast bowler can achieve this too, when he goes around the stumps, but no matter how close a right-armer tries to come to the stumps, he'll never be close enough; and when the ball is delivered from wide of the crease, as in these instances, it simply can't pitch within the stumps and still hit the wicket. That rules out one major mode of dismissal - the lbw. Also, the ball bowled by a right-arm fast bowler from around the wicket is unlikely to swing back enough into the right-hander: the angle is simply too acute. On the other hand, the left-arm fast bowler doesn't face any such issues since the bowling arm is a lot closer to the stumps, and the moment it starts swinging back into the right-hander, the problems multiply for the batsman. That is where Starc and Boult have scored over the rest.
If it's only angling away, you can manage by planting the front foot across and playing outside the line every time. But as soon as one ball darts back in you need to rethink and realign. Now you must not commit too much or else the incoming ball will have you in a tangle. A short front-foot stride is a must to the ball that's swinging back in. But the problem is that it's a given that some balls will hold their line and not come back in. The only way to gauge if the ball is going to swing or not is to keep an eye on the position of the seam and watch it in the flight to figure out if the seam is wobbling or not. If the seam is facing leg slip and it's coming down bolt upright, you ought to assume that it's going to swing in. Just that this, or any other method one might have, isn't foolproof. It happens for the right-arm fast bowlers too but the lack of angle there makes it easier for the batsman to handle the lack of swing on some deliveries; with a left-arm fast bowler, these balls often find the outside edge.
"If pitches across the world keep getting flatter and the one-day rules stay as they are, it won't be surprising if a left-arm fast bowler becomes mandatory in every starting XI"
Left-arm fast bowlers have another inherent advantage over their right-arm colleagues because the angle that they create buys them some time and keeps the batsman quiet. Even if the ball isn't swinging in the air and the pitch isn't offering anything, the ball going across the batsman ensures that playing on the up isn't as easy as playing through the line to right-arm fast bowlers. While facing a left-arm fast bowler, the ball is always going away from you, and that sows the seeds of doubt.
It goes without saying that about 80% of batsmen and bowlers in the world are right-hand, and so are used to batting and bowling against those of their kind. Familiarity breeds ease, if not contempt, in this case. Picture Wahab, who has a slightly tilted wrist at the point of release, which takes ten out of ten balls away from the right-hander, but still the angle is enough to fool even the best in the business. Remember Hashim Amla falling for one of those and nicking to Sarfraz Ahmed?
I also think that it's easier to set fields to left-arm fast bowlers bowling over the stumps, simply because playing across the line to balls bowled at that angle is flirting with danger. These days, getting hit is inevitable for a bowler, and the only way to minimise the damage is to make sure that you decide where the batsman hits you. For a right-arm bowler, it's almost mandatory to man both sides because anything finishing within the stumps can be hit through the on side, and if you bowl outside off, the option for the batsman to walk across opens up too. The only way for the right-arm fast bowler to stop the batsman from doing this is to swing the ball away from him, but this doesn't always work when the ball is old. A left-arm fast bowler doesn't have such issues, for nine out of ten times the right-hander will target the off side against them: since the angle is taking the ball away, it's prudent to play with it instead of against it.
Finally, their natural variations give left-armers a head start in the death overs too. There are only three lengths that should be targeted in the death overs - bouncer, yorker, and the back-of-good-length for the heavy ball. The first two when bowled by a left-arm bowler over the wicket are difficult to get away. You're taught as a batsman that one shouldn't hook or pull the balls that are going away from you, for it's tougher to drag them in and still control them. Yorkers also pose a similar threat if they are going away from you, for the main target area for the batsman in the death overs is midwicket/long-on, and to hit there, you must hit against the angle.
A lot of right-arm fast bowlers try to exploit that angle too in the final stretch of an innings, but when they do so, their slower delivery loses potency. The easiest slower one to bowl is the offspin variety, and for a left-arm fast bowler that delivery automatically goes away from the right-hand batsman, which isn't the case with the right-arm fast bowler: he must develop a back-of-the-hand slower one or a legspin variant, neither of which is easy to acquire. Also, since their bowling arm is so wide on the crease, these variations are difficult to execute properly. Starc is phenomenal in the death overs for these very reasons - first the angle, then the reverse swing tailing away at the end, and finally the offspin that leaves the right-hander.
If pitches across the world keep getting flatter and the one-day rules stay as they are, it won't be surprising if a left-arm fast bowler becomes mandatory in every starting XI.