By "polishing" an older ball (say 30-40 overs) on one side only, especially in damp weather, what happens is that the polished side has less air-friction and thus moves faster than the unpolished side, causing turn in mid-air.
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If you have slips next to the wicket keeper, all you need to do is point the seam to the closest to the keeper and you will get a late swing, if you point it to the furtherest away slip you will get an early swing
the swing is produced by the movement of the wind and completely depends upon the movement of the wrists of the bowlers.