Clark, I had a similar discussion recently but we were talking about moving the finish line using a spare mark boat. 

In that instance, the Flag was transferred to the new finish boat (to square the DW leg after a windward change-mark), but from the windward mark, boats can’t see which boat has the flag and point to boat whose outline they remember. 

My current practice is to use a repeated sound signal under the hypothesis that although the rule requires a two sounds one time, it does not expressly prohibit repeating the sound signals and it is difficult to envision a situation where repeated sound signals would be a redressable error.
I'm prepared for the eventuality that this practice would be frowned upon during my IRO exam, but I'm OK with that.

The beauty of this rule is that even if they don't see the flag, they finish. Of course, it would be better to know that it is happening. In light air, I usually have an assistant hold the flag out. Generally, I sound the horns when the first boat is clearly within earshot. The fact that they finish and sail away is a good clue to the rest of the fleet about what's going on. I like the idea of repeating the sound signals. How do you all feel about a VHF announcement after the sound signals in fleets that have VHF communication? 

You could always use a placard with the Sierra flag on it.  Or hold it out as John Porter recommends.

But I'm against using the Sierra flag if at all possible.  The only two races I've ever had to abandon after the fact were two races where I tried "to do the right thing" and shorten the course.  Once burned, twice shy.  A lot of one-design classes (and World Sailing at WS Cup regattas) forbid its use in major championships.

Clark,
Your premise is that competitors will somehow sail differently if they are about to round a mark as opposed to finishing.  But if the location of the mark boat is correct (at an angle to the last leg and on the outside of the course) there will not be any difference in tactics or direction.  I'm not saying it doesn't matter, but it doesn't matter very much.  That's why the rule is written as it is: there's no notice requirement.  Just get the flag up and start finishing boats.

I do agree that the sound signal and flag should be displayed when the first boat is quite close to the mark.

At our club, our typical 50 min (target time) race is eight or nine W-L legs. With a dying evening breeze, Sierra is very commonly used.
I can see why World Sailing would forbid it at WS Cup regattas, especially with trapezoid courses, but that's a very different kettle of fish.
Making the sound signal "when the first boat is clearly within earshot" doesn't address the problem when there is a significant gap between the first boat and the balance of the fleet.

The reason that I suggested "repeated until at least the first boat finishes.” is that if there are big gaps in the fleet, the RC boat will usually have little to do during these gaps and can easily make more sound signals if warranted. OTOH, I didn't think that it was a good idea (or necessary) to mandate repeated, potentially confusing, sound signals throughout the finishing process.

Instruct the RC staff to use a compressed air can pointed slightly towards the water to improve sound reflectivity and make the two sounds followed by a 3 second break, then 2 sounds, keep repeat until all boats get the idea.  If you want to be green about it, purchase multiple re-fillable air horn's per RC boat so that your RC staff can bleat away.  For small boat races I tend to use a metal whistle as it's sound travels very well.

I usually end up putting something like this in the SIs:
" To shorten the course at the leeward mark, the race committee will display flag S with two sound signals and will also position a RIB near the leeward mark displaying flag S and making repeated sound signals.  Boats must then leave the leeward mark (i.e. mark 4p or mark 3 [depending on the course]) to port and sail across the finish line.  This changes rule 32.2."

John Culter wrote:

I disagree.  On a mark rounding, there's often significant tactical value to sailing to the inside for overlap.  Gates mitigate that to some extent but not entirely.  As a result, boats sailing to cross a finish line as opposed to rounding might make different choices throughout the downwind leg.

I will be using John's SI concept for changing rule 32.2 for our standard SI's in 2020.  It prevents competitor mistakes.  Brilliant.

Remember the best place to hide info from the competitors is in the SIs.

The display of "S" should be made as soon as the decision is made with the accompanying sound signal using the largest "S" flag available since it may affect the tactics as competitors approach the new finish mark, especially if it is at a gate mark.. A back up announcement on VHF repeated at intervals also helps for the classes which allow radios to be used.  Most of the sound making devices used are easily heard especially if you are shortening a race due to lack of wind.

I find that most of the responses here do not give enough credit to the competitors. 
Most competitors can predict a competent RC will seek to shorten (or Change) a course when the conditions require.
A competitor who cannot anticipate likely action of the RC is unlikely to be leading a race, so generally not a concern. 
A good RC will announce its intention to, or action when posting a C or S. 
  

Some courses have the pin at the end of the line left to port if there is another round and left to starboard if finishing.

That has been tried and leads to confusion - much better to finish with he mark to be left as usual to port!

Repeated signals and a placard are good suggestions and the decibel level of the signals too should be high enough The spirit of any signal is the communicate equally and fairly to all competitors under the given weather conditions 

The shorten course signal, flag and sound, is 'proper' even though it is made when no boat can see or hear it.  Not very helpful though. 
My  area RYA Rules adviser (SE England) suggests there is nothing  in the rules to prevent race committees giving repetitive sound signals to a fleet which is so spread out that the original signal can not be seen/heard by most of the fleet.
This is very important to stop boats making a 'hook' finish when the course is shortened at a port rounding mark but for whatever reason the committee boat takes up a position to port so the mark becomes the finishing mark at the starboard end.
Using the S flag and sound signals at a leeward mark or gate when the race committee wants to shorten course so boats sail to finish at the finish line down wind is confusing.
It is better to signal 'C' over 'R' with repetitive sound signals, that is a change of course: 'round this mark and go to the finish line'.  yrs Simon

Should have said 'C over R' procedure saves having to positon race official recorders at the leeward mark instead of on the main committee boat.

32.2.b.2 says the finishing line shall be,

"a line the course requires boats to cross"
which may be the line defined by S and the rounding mark and stretching on to infinity.
It normally is, but does not have to be, the line segment defined between those two points.
A boat making the "hook" will have finished before hooking. 
A boat that interprets S normally and finishes "in the direction from the last mark" will also finish. 
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Edit:
SEE UPDATE/CORRECTION BELOW>

As a slight aside, in this past year's J/24 North Americans, which was held on an inland lake in Valle de Bravo, Mexico, the Race Committee used an interesting technique which we found to be quite effective.  The target time for a race was around 50 minutes, but, as we know, with a large fleet on an inland lake with no current and a high level of parity in boatspeed, a normal 1.1 mile beat makes for highly crowded first weather mark rounding.  

So, to remedy this, the R/C did something very interesting.  They set a very long first beat, around 1.4 or 1.5 nm.  Then, they shortened course on the final beat each race, down to say 0.6 nm.  They let the competitors know their intentions at the competitor's meeting, and in the end it seemed to work quite well.  I would encourage PROs to think about using this technique in certain conditions.

Hello Mays   What you report about shortening the final leg of a course is a change of course RRS33 which can be handled by race Signals 'C' plus '+' at the beginning of the leg, especially if the fleet has been warned in a briefing.  But the RC will still need to signal Shorten course if the fleet is to finish at the mark in its shortened position. 

"C" plus "-" at the leeward mark start of the shortened windward leg.
"S" (plus Blue?) at the new line, opposite the new mark in the color described in SI for new marks - or a whaler or other object flying "M."

Jeff H asked for clarification of my proposal of a shortened finish line to infinity.
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The RC may establish a shortened course finish line at any line that the course requires boats to cross.

The line does not have to be visible to the competitors.

A line of sight between the S flag and "that really big tree on shore" (and continuing 180 degrees out beyond the S flag to infinity) can be a legitimate line the course requires boats to cross on their way to what was to be the next mark.

My Judge instructor chuckled, but agreed with me.

The trouble is that competitors may not learn that they have finished. But the objective is to save a race that otherwise might not finish.

Best case is a line perpendicular to the course - and well before the next turning mark.

Setting up near the turning mark weakens the proposal and undoubtedly will lead to the protest room because of Case 129. The RC would have to claim that the turning mark was beyond the (invisible) shortened finish line.

So I was not quite right in my earlier posting. If the turning mark is involved, there is only one correct way to finish.