I'd say that the two rules are not ambiguous at all.
Yield when starting or entering - don't put someone who is already engaged in a situation where he has to do something dangerous to avoid you, and possibly put other people at risk. In France, that is formulated as : "the skier must always ensure that they can engage in a manner that puts neither themselves nor any other skier in danger" (replace skier by 'user' or 'client' or whatever you like). If you're stopped at the side of the piste, would you in all honestly start a big wide carve if there were a group of 'flat out down the hill skiers' coming, or would you wait?
The second puts other people in a position where they simply cannot avoid putting you in danger from above, as they cannot know that you are there.
Of course, both of these open up all sorts of legal ambiguities. They are all trumped, at least over here, by "The skier should not, by his behaviour or equipment, put into danger other users". Which is common sense, really.
Now, I get right snotty about putting other people in danger : just above where I work is a red slope that comes down directly into an area for absolute beginners. There are nets, signs, poles, god-alone-knows-what to warn the people coming off of what is effectively a world-cup class race piste to slow the hell down. And yet we have at least 3 or 4 broken legs per season caused by (and I hesitate to use the word) 'people' who scream through the baby piste at insane speeds[1]. People like this make me very, very, angry.
I should probably add that the worst offenders for this sem to be younger people on snowblades, who (in a sweeping generalisation) have absolutely no awareness of what's going on, or the danger they are causing. They are closely followed by the 'good' skiers (where 'goodness' is measured in thousands of euros spent on their gear, not in terms of actual ability, and who are aware of the danger but don't seem to think it matters because it's only poor people). Boarders, even complete numbnuts learners, seem to be more calm. Although there are a good few boarders who seem to think 'just under the big bump in the middle of a full-on-screaming-fast-red slope' is the perfect place for a spot of sunbathing. To each our vices, I guess.
I think if we're honest, we've probably all done things that put other people in danger at least once, I'm certainly not perfect, but it is beholden on us all to act in a relatively sane and mature manner. Very few people will come out and seriously say "skiiers / snowbladers are a menace to piste safety", but one snowboarder being irresponsible and the backlash starts.
Oh, and I'm generally quite a chilled guy, but I do take my job seriously. Too seriously, perhaps.
On the upside, resort opens in 2 weeks, we have a good metre of puff at the top, my boards are tuned, the boots are tweaked, and I'm waiting for my mate to give me back my AT skis. And we had 25cm of snow at 900m this afternoon. :)
Simon
[1] These people are generally allowed to remount the lift, spend 15 minutes going up, before being met at the top by the security boys who strip them of their lift pass and put them back on the lift to go back down. 30 minutes getting cold and the removal of your slope priviledges tends to calm you down.