I agree with your interpretation of rule 42.  The practice is not unknown in team racing and umpires need to be sharper than I usually am to spot it and penalise.  There might be a tendency to wait and see if they do it again.  Having got the advantage off the line, they probably won't do it again.  

If it clearly propels the boat, then yes it breaches RRS42. The problem for judges/umpires is catching them at it and identifying the culprit(s) especially in a large fleet

At a Laser World Championship, I raised this point. The problems are those of consistency in a big fleet and being fair.  Most of the boats now start like this, you follow the boats in so for those where you see the rock you are not in a position to see the bow and if the speed changes. It is thus difficult. Whilst we cannot be sure therefore if the speed changes we can all count so judges penalise the second attempt at a roll. This is the practice.  If you disregard just catching just the one boat if it "clearly increases speed" it is a breach.

Rule 42 .2(b) prohibits repeated rolling of the boat; a single roll that does not clearly propel the boat is permitted (interpretation Rock 2).

The Webster's definition of propel is, "to drive forward or onward by or as if by means of a force that imparts motion."

It's obvious that the movement is being done to accelerate the boat from a near stop up to normal sailing speed - so I would also agree with your interpretation.

However, the tactic is widely used and rarely called in my experience.  It's difficult to make the "clearly propel" judgement required when in the typical 42 judge position below the starting line looking at the sterns of all the boats.  The focus tends to be on sculling and pumping at the start.

Accelerating (by sheeting in) and luffing (with a heeled boat to leeward),  and flattening it (after reaching close hold) as part of the starting maneuver can perfectly fit to my understanding of "trimming the sails and hull and other acts of seamanship". It can be seen also as one pump and one roll. The critical moment as I understand it is the speed of the boat reached by those actions.  If the boat is able to maintain (if the wind and other conditions stay) the speed, then it was seamanlike. If the speed clearly drops after short acceleration, then the roll and/or sheeting in clearly propelled the boat  See the principal in RRS 42 Interpretation BASIC 7.  The speed of the boat can be tested by the hight of the bow wave easily also from the standard jury boat position - from behind the boat. Am I correct?

While 42.2 (b) talks about repeated actions, off the start line anything that infringes 42.1 is a tactical breach and so should be penalised.  In terms of what it looks like, there is a difference when 42.3 (a) is applied.  As we know, any boat will round up if heeled to leeward, but this isn't an instantaneous effect.  So, if a sailor stands up two seconds before the start and then sits heavily to level the boat at the signal, that isn't rolling to facilitate steering.  The sailor needs to have been seen to be standing and heeling the boat some time before the start to legitimise the action.  Or so I'm told by a far wiser and more experienced IJ/IU who is on the R42 committee.
As for judging whether the action propels the boat, on the start line I look at the relative positions of the sterns of the 'offending' boat and the one immediately to windward.
When it comes to 'they all do it so it's unfair to penalise one', I disagree.  If you don't penalise they'll all do it and consider it normal practice.  They have to think that next time it might be them.  And when they get to that all important regional, national, continental or world championship, with judges that apply the rule more carefully, it could cost them the regatta.  I have seen one young woman lose a world championship because (on a leg rather than the start) a red mist came down when she saw her brother ahead of her on the water.  I was less than 10 metres away when she started rocking to make up ground...

Ant,
If a sailor stands up 2 secs before the starting signal ( to heel the boat to leeward and luff) and then sits heavily to level the boat at the signal ( just after reaching the close-hold course) and by those actions the boat reaches the speed which is maintained further, I think, that no rule was broken. ... 2 secs can be just OK for certain wind and boat position.   

I believe that you are facilitating steering.  Flattening the boat helps bring the boat to close haul.so 42.3a allows it.  You are trimming, and hiking, to help steer the boat from a stop or luffing position.  I say seamanlike.  They are hiking to flatten the boat which is the most seamanship skill available for almost any class of boat.  Increased speed does come into play in a tack or jibe but we are not talking about that in this case.  I have PRO'd a lot of dinghy events and judged a few as well and not to be one of those guys but everyone does it.  IF it is repeated and flagrant it is a rules violation.  If it is one fluid, seamanship like motion I think it is legal.

This is allowed specifically in college sailing. It is strictly enforced in olympic racing (small fleets). If it is used to steer, it's more or less allowed, but if there's no steering it's technically illegal. Fleets should make a CLEAR rule that is easily enforceable. The biggest issue is when some feel free to "get away" with something and others aren't sure if it's legal.

Dusan and Ryan,
The Rule 42 committee at WS seem to disagree with you.  But, Dusan you raise an interesting point about 'certain wind'.  If the boat was already heeled, and the wind was blowing hard enough to warrant the action just to flatten it, I'd probably let it go.  If it was done in 'just enough to sail in' wind I wouldn't.  As in all cases, 'it depends' but as a general rule, the level of righting moment applied by body action has to be appropriate to the wind conditions.
Rock 6 interpretation states quite clearly that the heeling of the boat has to be consistent with the boat's turn.  So, if a sailor heels a boat (to leeward in this case) then there must be some turning to windward involved before the boat is levelled.  If the sailor continues to steer the boat in a straight line, or away from the wind and then uses the body to heel the boat that would very probably constitute a breach of the rule.  There is no repeated action on the start line, if the action clearly propels the boat forward relative to another boat that hasn't used the tactic, that is a tactical breach and should be penalised.

World Sailing's guidelines on Rule 42 are worth taking a look at here since they provide additional verbiage in addition to what's the RRS.

Below is some specific verbiage from the Laser guidelines, at the start. Summary: exactly one roll is permitted, unless it clearly propels the boat.

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STARTS 

1. One Roll and One Body Pump 

A single roll or body pump at the start is permitted unless it clearly propels the boat. Very often one roll is combined with a strong body pump at the completion of the roll and this may break the basic rule.

Permitted actions: 

• One roll or one body pump that does not clearly propel the boat 

Prohibited actions: 

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I'm assuming the thinking is that most dinghies will bear away to a close reach several seconds before the start to accelerate, sheet in, and then heel to leeward a couple of seconds prior to the start to facilitate steering upwind. 

My two cents, speaking as a Laser sailor:

To sum up, only case #1 is what I'd consider a rules violation. If I were a judge at a Laser regatta, I'd look for three things to happen to penalize someone for one roll at a start:

For what it's worth, the general practice for umpires and judges in college sailing in the US is to hang our hats on the basic rule 42, rule 42.2(b) and rule 42.3(a).  In a nutshell, if the heel to leeward and flattening doesn't propel the boat (doesn't break the basic rule) or can reasonably be interpreted as facilitating steering (42.3(a)) and isn't repeated (42.2(b)), it's OK.  In most cases, this comes down to the heel facilitating steering.  So we require that the boat must luff up when she is heeled -- a boat that heels, flattens, accelerates and then turns up breaks the rule.  According to World Sailing guidance, judges are supposed to determine whether the heel is commensurate with the turn, but in practice that's nearly impossible to do, and we simply require that the boat has to turn up while still heeled -- or, as actually happens, the crew sees they're about to break a rule, flattens the boat slowly and gracefully, and doesn't accelerate during the maneuver.