org/wiki/Short_of_a_length ), "back of a length" means the same as "short of a length", which I do know. As the article says, it refers to a ball that pitches short of the optimum length. "sliding in with the arm" is obviously describing the way someone moves, but, to be honest, I'm not sure if it's talking about the bowler or the batsman.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
User_gary
I feel "sliding in the arm" describes the movement of the ball thrown by the bowler to the batsman, but I can't understand the exact meaning of the movement.