Saw that coming, an easy one:

RRS 21.3: A boat moving astern by backing a sail shall keep clear of one that is not.

Interesting terminology. So, if you're in a ripping foul current, you could back down and have rights, as long as you're not back(wind)ing a sail???

As for the side discussion about "to park or not to park" the real question isn't about being REQUIRED to anticipate (by the rules, you do NOT have to anticipate, just react); the real question is, why would you wait until the boat is under you to react? To correctly use "parking" as a tactic, you need to vigorously defend, which requires anticipation, and pre-emptive action to block your hole.

Mike