Well the position of arm will be at an angle of somewhere between the 45 and 60 degrees with respect to horizontal shoulder level position as shown in the picture.

When the position of arm is maintained at an angle of between 45 and 60 degrees, it creates the flexibility for the bowler to rotate the ball from right to left with the help of ring or turning finger in the direction of slip region along with the seam, which actually will be the key in bowling legbreak