I'm over early and backing the main to reverse behind the line — do I keep my port-starboard rights?
No. Backing down is a legal fix, but Rule 21.3 says a boat moving astern, or sideways to windward, through the water by backing a sail shall keep clear of one that is not. And it goes further than a new duty: the Section D preamble says that when Rule 21 applies between two boats, the Section A rules do not. Port-starboard, windward-leeward and clear ahead-clear astern all switch off. Until you stop backing, you are the keep-clear boat — look behind you first.
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