Basically it is the desire of a pace bowler to increase the speed with every ball. Chucking increases the speed impact. Chucking can be natural flaw in the action or even deliberate. The best tip is keep your arm straight, continue the rythum once you start raising your hand towards delivery. At the time of delivery do not put any extra pressure on the ball. For classic chucking free action, see Denis Lillee, Wasim Akran, Sir Gary Sobers, Ian Boatham etc.