Greg Baum wrote this interesting piece on 30-July-2019 on the art and science of reverse swing.
Reverse swing, as the name suggests, moves in the opposite direction to conventional swing. Instead of the ball drifting towards its rougher side, it veers towards the smooth. Typically, a ball needs to be older, and thoroughly knocked about, before it arrives in “the zone” to reverse; and it takes a faster bowler to inject enough velocity that the effect will take hold.
In Simon Jones and Andrew Flintoff, England possessed two fast bowlers with the speed and skill to exploit reverse swing. But all the bowlers, spinner Ashley Giles included, bought into the plan to care for the ball as it aged, polishing one “slightly rough” side while allowing the other to become, as Cooley puts it, “super rough” from being hit or bowled into the abrasive pitch. The idea was to create a marked contrast between the rough and super-rough sides of the ball – one that primed the ball for reverse. As for the seam, Cooley says Jones held it bolt upright for reverse swing while Matthew Hoggard, also of 2005 Ashes fame, held it at a slight angle, as he also would for conventional swing, for which he was better known.
While conventional swing involves air hugging the surface for longer on the rough side of the ball, something peculiar happens in reverse. When a reverse-ready ball is delivered at a high enough speed, the layers of air that form on either side transition from what scientists call “laminar” to “turbulent” on both sides, but at different times. On the super-rough side, the air layer becomes particularly turbulent and thickens to the point that it separates from the ball much earlier.
On the relatively smooth side, the air layer is turbulent to a degree that means it will now stick for longer before separating. The uneven points of separation create an uneven wake behind the ball and a side force, as in conventional swing – only in the opposite direction. Jones confused Clarke by bowling a succession of reverse swing balls that swerved away, then surreptitiously turning the shiny side towards the batsman to produce a reversing inswinger.
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