From World Sailing , the Racing Rules of Sailing
28.2 A string representing a boat’s track from the time she begins to approach the starting line from its pre-start side to start until she finishes shall, when drawn taut,
(a) pass each mark on the required side and in the correct order,
(b) touch each rounding mark, and
(c) pass between the marks of a gate from the direction of the previous mark.
She may correct any errors to comply with this rule, provided she has not finished.
Occasionally, boats do not sail the correct course set for the race, either through misunderstanding or accident.
Having sailed the wrong course, these boats are not eligible to finish, and subject to disqualification.
However, the RRS provide an opportunity for skippers who realise their error, to rectify that error and Finish correctly.
“The String Rule”
Rule 28.2 is known as the “String Rule”.
Imagine your boat un-spooling a string from the stern as it sails around the course. That string should pass each mark in the correct order and in the correct direction.
After making a course or rounding direction error, the boat must return along its original track, “winding -up” the string until the error has been cancelled, and only then resume the correct course.
If the mark has been rounded in the wrong direction,(eg to port instead of starboard), the original rounding must be “undone”, and the correct rounding performed!
In effect, this means that, if a skipper realises the error soon after it occurs, the error can be corrected quickly and his/her race continue.
On the other hand, if the boat has sailed some distance since the error first occurred, there is little chance of rectifying the error and finishing competitively. Hence skippers generally acknowledge their error and Retire from the race.
Even after crossing the Finishing Line, a skipper who later becomes aware that their course was not correct, should Retire from the race by notifying the Race Officer - the honourable thing to do in our self-policed sport.
Boats who do not sail the correct course are liable to be protested by other competitors and/or the Race Officer/Committee.
Rule 28.2 errors are more likely to occur when there are many marks to be observed, and/or similar marks are laid in relative close proximity, but skippers are responsible for having a clear understanding of the course before taking to the water!