Murray - that's spin bowling, imparting spin to the ball so that when it bounces it changes direction.

Gary - right enough, polishing one side of the ball. That helps. So does the action of the bowler - side on or chest on. And cloud cover. And humidity. The trouble is that sometimes it does and sometimes it doesn't!

There also used to be a theory about some balls being more liable to swing than others. The old swing bowlers could tell a good 'un by smelling it. Sniffing your balls, an important part of cricket, that!