This grommet, invention of Mr Cunningham is a revolution.
It allows us to adjust the sail along the mast with a fixed gooseneck
The cunningham creates a vertical tension on the luff of the sail. Its first interest is to correct a relative under tension of the luff compared to the leech. This under tension generates characteristic wrinkles upright to the mast.

As a matter of fact, we can do the following experience: Cunningham released, pull in the boom vang or sheet in trying to have the pulleys close one to each other. Immediately winkles appear from the luff to the outhaul. Pulling the Cunningham, they disappear.

But it is not the only role of the Cunningham. Its essential roles are:
 

• To bend the mast in complement of the boom vang and the main sheet. When the mast is bent, the vertical tension due to the Cunningham is a little pushed out of the back of the mast. This tension, by “bow effect” increase the bending of the mast and help the effects from the boom vang or/and the sheet.
   
• To push the depth of the sail to the mast, or at least, prevent it to go to the outhaul. When the wind increases, the depth of the sail goes to the outhaul, with a very bad effect on the aerodynamic force orientation. The Cunningham allows, pulling the fabric to the mast, to better center the depth in the sail.
   
• To relatively loosen the leech with an over tension of the luff. This allows the upper part of the sail to go outside in a squall, minimizing the aerodynamic force. This twist creates some of auto-regulation of the squall only with the rigging.