Once you've cracked getting the ball down to the other end of the pitch, it's time to start thinking about whether you can extract any movement from the ball on its way down - after all, it's much easier for a batsman to hit a ball that goes in a straight line than one that deviates. Swing bowlers can be equally devastating and out-and-out speed merchants when the conditions suit them; traditionally humid, moist and overcast conditions have always increased the amount of swing that all bowlers can achieve - nobody's quite sure of the science behind this, but the evidence is there for all to see. Personally, the most difficult batting conditions I've ever experienced came during a net practice beneath skies leaden charcoal grey in the few minutes directly before a heavy cloudburst - every bowler I faced found extravagant movement.
Whilst the conditions will always be a major factor in how much swing a bowler can get, there are several things he can do with his action to maximise the amount of movement that he can get, no matter what conditions he experiences. Firstly, he must look at the type of action (see the previous section) that he feels most comfortable with, as the angle of the body as the ball is released has a great bearing on the type of swing that can be attained. A 'side-on' bowler will find it much easier to bowl away-swing (for a right handed bowler, moving the ball away from a right-handed batsman) because of the path of the arm in the action, whilst a 'front-on' action will make in-swing (for the right-armer, moving the ball back into a right-hander).
To build upon the natural action, a bowler can point, angle or 'can't' the seam of the ball in the direction he wants it to swing. Staying with the example of a right-handed bowler bowling to a right-handed batsman, for an out-swinger the seam would point towards the slip cordon, with the side of the thumb on the seam at the bottom of the ball. The seam on an in-swinger would aim towards fine leg - this time, the bottom of the thumb would lie along the seam at the bottom of the ball. Next, the shiny side of the ball should lie on the opposite side of the direction in which the movement is wanted - once again, there's some simple science behind this. A smooth, shiny surface will move faster through the air than a rougher surface, and on the cricket ball this means that one side moves faster than the other. The shiny side moves faster through the air, and helps to increase the movement.
Reverse-swing is a phenomenon that's become much more present in world cricket in the last decade, mainly thanks to the Pakistani pairing of Wasim Akram and Waqar Younis. In its simplest terms, it involves the shiny side no longer being the fastest through the air, but the rough side. For this to happen, the rough side needs to become exceptionally scuffed and the ball needs to be delivered at high speed - often in excess of 85mph (140kph).
All three types of swing have one thing in common, however, in that they need the seam of the ball to stay straight throughout the delivery - if the seam is scrambled, the smooth and rough sides will not stay the same and so the effects won't occur. To keep the seam straight, the wrist needs to be kept straight as the ball is delivered. It can also help if the wrist is 'cocked' - or bent backwards - slightly before release; this helps to attain rotations on the ball which in turn, as they cause the ball to rotate forwards over itself, reduce any sideways spin that disrupts the seam position.