The spin vs skid above has an analogy in the ‘wobble-seam’ that Broad and Anderson use sometimes. It introduces an element of natural/random variation that the bowler can’t predict, so nor can the batsman. Half a bat’s width is often all the movement you need.
Apparently Shaun Pollock could naturally cant the seam a few degrees over from the vertical and thus ‘seam’ the ball one way or the other intentionally. Wobble seam achieves similar, but much easier.
Traditionally finger spinners had an ‘arm ball’ that was effectively a swing delivery pushed off of the index finger. But it is relatively easy to spot if you’re watching the ball closely. A better disguised variation is what jon describes above – just pushing the ball out without quite as much over spin means it might pitch on the flat part of the ball and skid.
The finger spinners who had the best arm balls tended to be the ones who got very side on and put a lot of ‘body’ into their action (Robert Croft, John Emburey, Tim May). Spinners like Ashwin and Swann ‘rip’ the ball harder with a front on action, meaning the ‘skidder’ is a better option than an arm ball.
