In swing

In-swing is a popular cricketing trick used by bowlers of all classes the world over to confuse batsmen. It is difficult to spot until the very last moment, and the path of the ball is always unpredictable even for the bowler. This double-edged sword means that the bowler has to be fairly confident of his line and length. If he does not calculate correctly, or either under/over compensates for the effect of the swing (movement towards the leg side], the ball will likely go wide. If he is underconfident and bowls too slowly, the batsmen will dispatch him to the boundary. Here, have some diagrams:

A 'stock' (normal) ball:

      ^
   ---^- 0-0-0 -----
   |  ^             |
   |  ^             |
   |  ^             |
   |  ^             |
|__|__^____X________|__|
      ^
      ^
      ^
      ^
      B
      ^
Flightpath of    Bounce: B
normal ball: ^  Position of batsman: X
 

A ball delivered with in-swing:

      
   ---- 0-^-0 -----
   |     ^         |
   |     ^         |
   |    ^          |
   |    ^          |
|__|___B__X________|__|
       ^
      ^
     ^
     ^
    ^
    ^
Flightpath of    Bounce: B
swung ball: ^  Position of batsman: X 

To bowl in-swing, several factors come together. The ball itself ought to have one shiny, one rough side. The shiny side must be on the outside, i.e. the shiny side moves faster due to less air resistance, thus the ball swings. That's one factor. Officially, science cannot explain why the ball swings. Consistant swing is definitely best achieved in hot and humid weather. Certainly grip is important, too. The most conventional grip has the index finger on the seam, thumb underneath, second resting on the leather with third and fourth fingers tucked under the ball. If you flick your wrist downwards at the moment of release, you'll also get a fair deal of off-cut too, i.e. the ball will move even more in the same direction after the bounce as you've imparted spin to the seam, which is a raised ridge of stitching running around the circumference.

The most important factor in the bowling of in-swing, however, is the bowler's action. This is the manner in which one releases the ball. You can get a yard of in-swing with the ball the wrong way around and your grip wrong in cool crisp weather if your action is good enough. Essentially, you have to release the ball facing the batsman front on. You still have to jump side on, non-bowling arm raised, but you have to twist your spine round to face the perpendicular of the direction of the ball's travel.

What's the damn point? Well, major proponents of in-swing like Darren Gough use it in conjunction with a yorker. This severely unsettles the batsman. What had been another pedestrian ball is now not only about to hit his stumps, but also hit him on the feet. In-swinging yorkers are amongst the nastiest things in the world that can approach you at 93 miles per hour. This ball will either hit your wickets because you're running away from it (seriously, people do back away) or because you've entirely missed the fact that it's swinging, played outside the line and been similarly bowled. Or, it could hit you on the legs and have you out LBW. See the Rules of Cricket for details on this. The capability to bowl in-swing is one of the most potent weapons in a fast bowler's armoury.