The main cunningham controls the fore and aft position of the draft in the sail. Keep it loose until you begin to be overpowered and then tighten it to open the leech of the main by moving the draft forward in the sail.
The Mainsail Look When the Mast is Properly Bent: On all masts we are looking for very slight overbend wrinkles (diagonal wrinkles that will appear from the spreaders to the clew) showing that the mast is beginning to reach maximum mast bend. In light to medium wind these wrinkles should barely appear. In winds above 15-18 mph, these wrinkles will be quite noticeable and in above 20-25 mph winds, will be quite severe. If these wrinkles are not developing, your mast is not bending enough.