Mike,
Here is the formal wording of rule 17.1.
<i>17.1 If a boat clear astern becomes overlapped within two of her hull lengths
to leeward of a boat on the same tack, she shall not sail above her proper
course while they remain overlapped within that distance, unless in
doing so she promptly sails astern of the other boat. This rule does not
apply if the overlap begins while the windward boat is required by rule
13 to keep clear.</i>
What it means is...
If you are overtaking from clear astern and begin to pass a boat to leeward, and your bow crosses an imaginary line from the weather boats transom, you are no longer the over taking vessel and you then become overlapped and the leeward boat. The rule is designed to protect the weather boat from an immediate tactical move by the now leeward boat who might want to force you up and away from "proper course". The weather boat must keep clear but you are not allowed to sail above proper course.
On the otherhand, if you are overtaking and pass to weather, the leeward boat does have the right to defend.
Chuck
#84