The Downhaul, or Cunningham as it is also known, is used to to flatten the sail by pulling down the sail bolt-rope and, due to the curved cut of the sail at this point, it will cause the sail to flatten and bend the mast, thus reduce the fullness at the front of the sail and de-powering it. The 'A' Cat has a very powerful downhaul, in many cases 6 or 8:1 and multiplied by another block on the sail tack, making it 12:1 or so.  Small adjustments can have an effective result.

​The Mast Rotator, or Spanner as this is know, rotates the mast relative to the centre line of the boat.  As the mast of the 'A' cat has a side area of 1.5 sqm, it is important to get the angle of attack relative to the wind correct as this controls the airflow over the sail, rather like the leading edge slat on an aircraft wing.  The more the spanner is rotated aft, the flatter the sail will become.  This is another powerful tool to control your sail power and again, small adjustments have a noticeable effect. 

Articles

Replacing the Downhaul Pulley - Charles Bueche.  

An explanation of how to do it.