A sail is said to be luffing when the wind passing over it is not generating power to propel the boat forward. This may happen when a boat is sailing close-hauled as a result of turning the boat too close to the direction of the wind; or if the sheets are allowed to go slack when sailing any point of sail.
The loss of forward motion can be gradual or rapid depending on how severely the sail luffs.
The default key for allowing your sails to luff is the ?? however this can be bound to any other key.
If you are racing with manual sail trim then your sails may luff if the apparent wind angle (AWA) changes and you do not adjust either sail trim or boat heading to compensate.
Close hauled Edit
This is a point of sail in which the sails are set as close to the boat's center-line as the sheets or running rigging will allow and the boat is steered as close to the direction of the wind as possible without luffing.
A sail is said to be back-filled when the crew and not the sheets (or running rigging) hold the sail in such a way as to propel the boat backwards. Of course the reason for doing this is more about propelling the bow of the boat in such a direction as to allow the boat to proceed on a favorable tack and rarely to actually propel the boat backwards.
In VSK your boat may experience backwards motion (due to stream or wind or collision with another vessel) however, it is not possible to instruct your VSK crew to back-fill either head-sail or mainsail.