The arm ball, or floater, is probably the single best weapon an orthodox finger spinner can have. Ray Illingworth says he once took 41 wickets of 135 in a season just with the arm ball.
At first the ball seems counterproductive. Spinners should spin the ball hard, hoping to impart enough revolutions on the ball for it to dip late in its flight and move off the pitch. This is true for the stock delivery and is what you should do at least 80% of the time.
The arm ball gives you the perfect suprise variation from this.
The batsman is expecting the ball to spin as normal but you have bowled a ball that swings in the opposite direction with almost no change to your action: The off spinner's arm bowl swings away from the right hander, the left arm spinner's swings in.
This brings in the possibility of deceiving a player. Getting him bowed or caught at slip (in the off spinners case). In short, a true wicket taking delivery.
How to bowl an arm ball
The key difference between the arm ball and the stock ball for a finger spinner is the grip. The basic grip looks like this:
As you already know, this grip allows you to roll your fingers and wrist around as if turning a key in a lock. This imparts spin on the ball. The more you 'rip' it, the more spin is possible.
The arm ball is best bowled with no change of action, however your grip changes to one that looks like this
This allows you to swing or drift the ball using the seam as a rudder in the same way as a medium pace bowler. However you still look like you are bowling a stock spinning delivery. The shiny side of the ball should be on the leg side if you are an off spinner.
While you no longer use your fingers or wrist to impart turn, according to Ray Illingworth, it's still important to follow through in the same way. This is mainly to trick the batsman, but it also helps the ball swing.
Line and length for an arm ball
The arm ball is a shock wicket taking delivery. As a result you should be prepared to take more of a risk with it by pitching it further up. This makes the ball hard to sweep (bringing in bowled and LBW dismissals) and tempts the drive.
The line for off spinners to right handers is best on middle stump. If the batsman plays for spin he will edge it or be bowled.
The line for left arm spinners is outside off stump, swinging it back in to bowl the batsman through the gate.
Using the arm ball
It's important not to overuse this variation as it loses its element of surprise. Your main variations will come from the amount of flight and turn you impart on the ball.
However, used now and again you can have great success. It's especially effective early in a batsman's innings. You can use it on almost any kind of wicket, but if conditions do not suit swing think very carefully before trying it.
It certainly will not allow you to rip through an opposition order in itself, but when used in combination with excellent control can put doubt in a batting team and give you the chance to bowl a side out.
Photo credit: Alister667