If a bowler sends down the same sort of delivery every time that he runs in to bowl, the batsman can often get used to this and as a result find the bowling much easier to play. As a result of this, it's useful for a quick bowler to develop a 'variation' in order to prevent this kind of situation from occurring. A variation can come from a change as simple as slowing down the run-up, but there are several other, less obvious, ways that a bowler can change the type of delivery he's bowling - after all, if you're trying to surprise the batsman, you're much better off if he can't tell that you're doing something different!
As mentioned earlier, if the seam of the ball doesn't stay straight (or is 'scrambled'), then the ball won't swing as much - so by simply holding the ball with your fingers across the seam, you can reduce the movement that you get. It's also possible to slow down the delivery by changing the way that the ball is gripped - again, there are many different ways that this can be done, ranging from simply increasing the distance between your index and middle fingers on the seam through to the 'reverse release' made famous by Ian Harvey.
This slower ball, also known as the back-of-the-hand ball, involves twisting the wrist at the point of release so that the back of the hand faces the batsman, slowing down the release and hence the speed of the delivery to great effect. A well-disguised ball like this is often at its most effective in the final overs of a limited-overs game when the batsman are attempting to hit everything out of the park, and slowing a delivery down by that 10-20mph (15-30kph) can deceive the batsman enough to make the difference between the ball clattering into the pavilion roof and the middle stump being knocked out of the ground!
Due to the significant differences in actions between in-swing and out-swing, it's very hard to disguise one type of delivery as the other. Martin Bicknell, of Surrey and England, is one of the few people who can do this - in the Test Series against South Africa during summer 2003, he bowled two outswingers to Jacques Rudolph that were both left well alone. The next ball pitched in exactly the same place only to swing in the other direction and remove the batsman's off-stump. Whilst for many bowlers this kind of scenario is an impossibility, deliveries called 'cutters' make up the next best alternative.
Cutters involve the bowler placing spin on the ball so that it rotates in the air, and then moves off the pitch in the opposite direction to the batsman's expectations. This can be done simply by rotating the wrist as the ball is released - as it leaves your hand, it will then begin to spin sideways over itself in the direction that your wrist rotated. The amount of movement can be increased with the fingers helping to increase the speed of the rotation of the ball. For a right-handed outswing bowler, the most common variation is the 'off-cutter', achieved by rotating the wrist and fingers in a clockwise direction and moving back in to a right-handed batsman, whilst an inswing bowler is likely to find the 'leg-cutter' - which moves away from the right-hander and is attained by anti-clockwise rotation - more useful.
Of course, these different variations can be mixed and matched depending on what you feel is the best for the situation, and are an important part of a fast bowler's armoury when it comes to getting batsmen out. Like any part of cricket, some will work for some bowlers but not for others - so don't be concerned if one particular method doesn't work for you, there's almost certainly one that will!