I'm not an official or high level competitor, but I'll try to hypothetically describe a  situation I have experienced a few times in both the port and the starboard boat.

Wind 10 knots
Red keelboat is on port tack fetching an upwind finish.
Green keelboat is on starboard tack on course to cross red.
Red watches green at a distance of 100 meters and reasonably estimates that green will beat red.  At a distance of 80 meters (10 seconds later), red reasonably estimates that boats are on collision course. At 60 meters (10 seconds later) red reasonably judges that red will beat green.
Red continues on course and passes ahead of green by 20 meters.

Green says that at 80 meters out it had a "genuine and reasonable apprehension of collision" and therefor  felt it "needed to take avoiding action."  

Red says "No, we were watching. You need to get a lot closer to the give-way boat before you can be scared of a collision. In fact, the boats were continuously changing speed throughout the episode.  Your job is to sail ahead longer than you did."

So, question, what is the standard for judging a "genuine and reasonable apprehension of collision"? If skipper of red is an expert, does red need to tack or duck far out (and lose his fetch) because green may be a novice skipper and may get scared easily?