An arm ball is a type of spin in cricket that is usually bowled by an off-spin bowler. The arm ball is employed when the bowler feels that the batter has got comfortable on the crease and is easily able to read the spin pattern of the bowler.
The arm ball is quicker than a usual off-spin. The grip pattern is such that upon release, a backspin is inculcated on the bowl. The release is also done in a way that the bowler expects the bowl to swing a little. It is advised to land this bowl in front of the stumps in order to trap the batter leg before wicket. While the line should be straight, the length should be fuller in order to give the batter little time to think after the ball pitches. Moreover, since the amount of spin on this ball is minimal after pitching, it leaves the batter guessing as to where the ball will pitch.
The arm ball is named so for the fact that it does not spin much. Rather, the direction of the ball is in the same direction of the arm from which it was released.
In case of left arm bowlers the arm ball will move towards a right-handed batter and for the right arm bowler, the ball will shape away from the right-handed batter. Daniel Vettori, Harbhajan Singh, Graeme Swann and Rangana Herath are some modern bowlers who has great command over the arm ball.