The Laser cunningham (or downhaul) is located at the front of the mainsail, and is simply a small diameter rope that is run from an eyelet in the mainsail, with a few loops or purchases, down to the deck, then to a cleat.
How Does The Cunningham Work?
Sails are designed so that the curve in the sail moves toward the area of most tension. So when it is blowing and you have the vang on, the tension in the sail is towards the back, meaning the curvature in the sail moves aft, which is not optimal.
So to counteract this, by pulling down the cunningham and creating tension down the front edge of the sail (or the luff), it moves the curve forward of aft, flattening out the sail. This is critical when you need to depower the sail.
Setting the Cunningham
In light breezes, you should have the cunningham eased right off so that it just takes out the creases in the luff. As a rough guide when sailing upwind, as the wind increases, gradually pull the cunningham down, until at about 20 knots, the cunningham eyelet is at the top of the boom.
A loop (eg. a bowline) in the end of the cunningham is recommended so that it provides an easy holding point.