If you want to
shorten course, because, for example, you see new wind coming and you
want to end this race and get a new and fairer race started, you can do
it at the windward mark or at the leeward mark.
Just station a boat to the right of the mark and have it raise
an S-flag with TWO SOUNDS. (There
is no need to warn the sailors at the previous mark.)
If you are using a leeward GATE, set up very very near one of the gate marks because the finish line is between those two marks. This is new with the 2005 changes to the racing rules of sailing. Previously, to finish boats had to sail between the RC boat with the S-flag and the nearest gate mark. That's why setting up right next to one of the gate marks will avoid getting a sailor in trouble who knows the old rule but not the new one. The new rule is good because even sailors who don't realize you have shortened course, will sail through the gate and be (unconsciously) finished.
You cannot shorten
course in the middle of the course where the start and finish pins are