I hadn't chimed in yet because I thought mbounds had already given a correct and complete application of the rules (referring to an ISAF case as well). Citing the match racing call book can be problematic in the context of fleet racing, but I didn't see any conflict in this instance. Would a step-by-step analysis help out?

A boat is not generally required to anticipate the actions of another boat, just to react to what she is doing as she does it. That reaction has to be "prompt", and "seamanlike". Those terms, unfortunately, are a little vague, so I can't give you a definitive answer. The advice one of the senior judges in my area gave me was to substitute the phrase "without delay" for "prompt". That is, "prompt" does not mean "immediate", but it does mean no stalling. The term "seamanlike" is harder to interpret. At the SAYRA Judges Committee meeting last year, we had an hour-long discussion of what skill level "seamanlike" construed. We didn't reach a consensus. Some judges felt "seamanlike" meant expert, some average. My own opinion is that it means competent. That is, you are expected to handle your boat competently. If the boat can reasonably do something, you should be able to make her do it.

That said, when an overtaking boat becomes overlapped to leeward (gaining right-of-way), the windward boat is expected to act to keep clear (if necessary) competently and without delay. If the leeward boat heads up (changes course), then the windward boat must again act competently and without delay. If that entails sheeting in to build speed and gain steerage, then W must do so smartly.

I hope that helps,
Eric