How do bowlers actually get the ball to reverse? What conditions do they need to get the ball to swing and what are some of the tactics used to get the ball to swing?
Reverse swing is the swing a bowler gets with the old ball towards the shiny side of the old ball rather than the conventional swing a bowler gets. If a bowler is bowling an outswinger, reverse-swing makes it an inswinger and vice-versa.
What is the science behind reverse swing?
There have been plenty of theories about the science involved in reverse swing. The simplest explanation from former England bowling coach Troy Cooley is:
"Reverse swing is all to do with the deterioration of the ball and the seam position in flight. As the ball becomes rougher, it will take on a different characteristic as it deteriorates. So if you present the ball as an outswinger, the ball has deteriorated so much on the rough side that it takes on the characteristics of the shiny side. Which means a natural outswinger will become an inswinger and conversely, an inswinger into an outswinger."
Air passing over a cricket ball creates turbulence, and two kinds of swing result from the ball responding to surface turbulence in opposite ways. In conventional swing, the ball is believed to move towards the side of greater turbulence, while in 'reverse' swing it goes the other way, away from the turbulence, and hence the term. Differential turbulence between the ball's two halves can be created either by angling the seam (as in new-ball or conventional swing), or by allowing one half to scuff up through wear and tear while the other half is kept obsessively polished and smooth (as in reverse swing). If there is a pressure difference between the two sides of the ball a side force or swing will be generated.
How do bowlers actually get the ball to reverse?
There is nothing much to it, a bowler bowling outswingers will still have the seam pointed towards the slips, but with the rough side facing the batsman, instead of the smooth for conventional swing, the ball will now behave like an inswinger and swing into the batsman. So, if the contrast in surface roughness on the two sides of a ball is successfully created and maintained, just by keeping the rough side of the ball on the off-side, the shiny side on leg-side bowler can obtain reverse swing.
The grip, raised seam of the cricket ball, wear and tear on the ball, speed of the ball bowled (above ~80mph) and fast movement of the body (bowler’s action) is key to get the ball to reverse.
What conditions do they need to get the ball to swing and what are some of the tactics used to get the ball to swing?
Basically the condition needed to get the ball to swing is to polish one side of the ball and let other side to roughen during the course of play. The rough side should not be repaired and the shiny side must be properly shone by the players. The subcontinent pitches, which are dry and rough are considered to the best for reverse swing as the ball gets worn up quickly on a rough and dry pitch. Reverse swing favours the older ball but depending on the type of ball, manufacturing techniques ball may start to reverse even after 20 overs, usually occours at the end of innings (around ~40 overs).
Regardless of the chosen procedure for polishing, at the outset, the opening bowler should pick the side on the ball with the smaller or lighter (less rough) embossment and continue to polish only that side during the course of the innings. The other (seam) side of the ball should be allowed to roughen during the course of play to aid the production of reverse swing. Once the seam side has roughened enough, reverse swing is simply obtained by turning the ball over so that the rough side faces the batsman.
Reverse swing comes into play when the ball gets older, you may have seen bowlers cleaning a partial surface(side) of the ball. This Is done to enhance the reverse swing later in the innings. After a extended period of bowling, when you look at the ball, you will notice a rougher and shiny side, now the ball starts to swing in unconventional way.
Under a normal condition when the seam is towards the slip, then it is a outswinger, but in reverse swing it becomes inswinger, because of the airflow difference between the rough and shiny side. You could watch this video for further explanation.
If the pitch is dry then the ball becomes rough in a small amount of time (sub continent pitches)
Seam position and the angle also plays an important role in reverse swing
From the science perspective, it's the Magnus effect that causes the reverse swing, You have a ball with two layers a rough and a shiny one, try passing smoke onto two sides of ball,you will see that rough side produce turbulent flow, the shiny side produce laminar flow, this creates a pressure difference (Bernoulli effect) and makes the ball go the other direction than the batsman anticipated.