"There is a slight difference in the grip of the arm ball compared to the off spinner. The grip allows the seam of the ball to come out straight, so the ball may swing or seam. The arm ball should be used when you think the batsman is starting to find it easy to pick your deliveries.
The arm ball should surprise the batsman and make them play a false shot. You should be looking to pitch the arm ball on the stumps to enable LBW and Bold to be the main form of dismissals. As with the off spinner keep your feet aligned towards your target. And on release drive your weight towards your target. Drive your fingers over the top of the seam, so the seam comes straight down at the batsman.
This will create some swing and seam on the delivery. Remember this ball is used as an element of surprise. So don’t over use it."