Former England women's coach John Harmer gives the Academy the lowdown on what reverse swing is and how to bowl it.

In simple terms the ball will only change its flight path to the batsman when the air around the surface of the ball travels at different speeds.

Remember the ball is pushing air to the side as it travels forward. Why? Because the air must go somewhere.

Check out why aeroplanes take off or why yachts go forward into the wind! Or let a piece of paper drop from your fingers and then blow over the bend of the paper.

See the paper come up? This is because you have fast air on one side and slow air on the bottom side and the paper is "sucked" into the faster air current or flow.

The cricket ball does the same thing. A bowler creates this different speed of air on either side of the ball by using the seam or surface of the ball.

These are known as the shiny side and the rough side.

The ball must be 45 or more overs old before it will reverse swing.

When the ball is five or more overs old, the bowler decides which side he/she will polish as it is still quite smooth.

The opposite side is left to roughen up under normal wear and tear of the game. Air likes rough surfaces and flows quicker around that side.

Therefore suction occurs and the ball moves left or right in its flight.

Eventually the ball gets very old and the air changes it preference for the side of the ball that it will go faster around.

The rough side is now too rough and the once shiny side has arrived at a degree of roughness that the air prefers to act upon.

So without changing the position of the seam, the ball begins to swing in the opposite direction so called "reverse" swing.

The seam hasn't changed, but the air has changed its preference for the surfaces of the ball.

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