The Cunningham is an adjustment used to tension the luff of the mainsail to move the draft forward, offsetting the wind’s efforts to push the draft aft. Most mainsails come standard with a Cunningham cringle on the luff above the tack. This way you can rig a purchase system to the cringle to add tension to the luff of the sail.
As the wind picks up, the draft of the mainsail gets moved aft by the pressure of the wind, which reduces the effectiveness of the sail’s aerodynamic shape. Trimming the Cunningham maintains proper draft position. If the wind lightens, simply ease the Cunningham.
The Cunningham is named after the first person to create this adjustment, America’s Cup sailor Briggs Cunningham. A slang term for the adjustment is the “Clever Pig.”