The following article is an extract from "Bob Woolmer on Bowling" a coaching book from the late Bob Woolmer; former Pakistan, South Africa and Warwickshire coach. Published with permission.

The perfectly plotted and executed in- or outswinger, curving late in its flight and taking the edge or ducking back into the pads, is one of the most exhilarating experience’s a bowler can ever hope for. When bowled well, with good control over line and length, it can be devastating, as the batsman is deceived into playing one line, only to have the ball swing off that line, leaving him groping.

The physics of the swing can and the reverse swing are discussed in detail in the section on the science of the swing on pp. 69-84, but it is important at this stage to have a basic understanding of why a cricket ball veers off its original line under certain conditions.

The ball is made of four quarters, separated into two halves by a proud (protruding) seam. Coaches who explain swing often refer to the seam as a rudder, but it is far more like the prow of a boat, ‘splitting’ the air in front of the ball on its flight; and the two streams of air passing over the two sides of the ball travel at slightly different speeds. The side that is smooth and shiny will present less resistance to the air than the rough side: the resulting low pressure on that side of the ball will pull it in that direction and away from the high pressure. This manifests itself as a swing.

This is why bowlers – and fielders – endlessly polish the same side of the ball. Indeed, it is the entire team’s responsibility to ‘groom’ the ball in this way whenever it comes their way, dusting off bits of grass or mud that might be clinging to the smooth side, and shining it as vigorously as possible. (Before lashing on the spit and sweat, remember that seamers prefer not to let the smooth side get too damp. Swing bowler, on the other hand, need moisture to clean it and keep it shiny. Check what the bowler and the captain want in terms of ‘ball-grooming’ before you over-enthusiastically soak the ball!)

A prominent seam and a polished hemisphere only go so far. To swing the ball consistently and well you need:

• A good basic action, with the ability to vary it slightly, depending on the delivery you want to bowl.
• A good wrist position
• Subtle differences in grip
• Subtle variations on when you release the ball.

Dennis Lillee, who could swing the ball with the best of them when the situation demanded it, was adamant about what a good swing should do, and his explanation is an excellent starting point:

The most critical part of swing bowling is the way you let go of the ball from the hand. If this is not done with a high degree of precision, the ball will not swing at all or will swing only a little and too early in its flight to be any great danger to the batsman. It starts with the grip of the ball, which should be made by contact of the index and middle fingers on the top of the ball and the thumb at the bottom. This contact should be towards the tips of the fingers and the thumb (what we call ‘fingering’ the ball), because if the ball is gripped too deeply in the hand, the critical control needed to send the ball away correctly may be lost. The hand should be directed behind the ball at the point of delivery and must not undercut on either side. The ball is sent away with a natural under-spin, and I believe the more under-spin imparted on the ball the later it will swing. The seam should remain vertical throughout the flight down the wicket (1982).

His view that under-spin or back-spin is crucial to swing bowling was illustrated by Australian swing bowler Bob Massie, who took 16 wickets in his first Test at Lord’s in 1972. Lillee points out that when bowling, Massie managed to keep the seam absolutely steady for its flight, in no small part due to the large amounts of back-spin he put on the ball with a very whippy wrist action.

There is simply no better ball to bowl at a new batsman than a fast outswinging Yorker, or perhaps an outswinging half-volley on off-stump. This is a delivery that should be mastered by any bowler who wants to succeed in the game.

Tactically, the outswinger is intended to drag the batsman wide of his comfortable hitting zone, and to have him caught in the slips or at gully. However many young or inexperienced bowlers make the mistake of getting carried away by the swing, and effectively bowling at the slips! This gives the batsman a pleasant over or two of being able to leave the ball and get settled. The ideal outswinger should be hitting off-stump or just curving away past it. The straighter you start it, the more chance you have of pinning the batsman in front with an LBW shout, especially if he thinks it’s a straight ball and tries to work if off his pads.

For a right-arm bowler facing a right-arm batsman, the seam is angled towards first slip, while the fingers point down the wicket. Remember that the ball will swing towards its rough half, so in this case, the rough side faces cover. The wrist is angled in towards the body and cocked backwards, while the side of the thumb rests on the seam of the ball.

Outswing starts from ‘behind’ you. It takes some time to get the right feel of when to release the ball for maximum effect, but basically the ball needs to be released from fractionally behind the ear. The hand and body must stay on line – driving through towards the target – and follow –through must be full and complete.

This extract has been taken from Bob Woolmer on Bowling published by New Holland, ISBN: 9781847737502. If you are in the UK you can qualify for a 20% discount by clicking the above link and entering the code "Woolmer10"