...from the Case Book for 2005-2008.
Rule 17.1, On the Same Tack; Proper Course
CASE 7
When, after having been clear astern, a boat becomes overlapped to
leeward within two of her hull lengths of the other boat, the windward boat must keep clear, but the leeward boat must not sail above her proper course and must initially give the windward boat room to keep clear.
CASE 14
When, owing to a difference of opinion about a leeward boat’s proper
course, two boats on the same tack converge, the windward boat must
keep clear. There can be more than one proper course.
CASE 46
A leeward boat is entitled to sail up to her proper course, even when she has established a leeward overlap from clear astern.
It all has to do with proper course...<b>the leeward boat may not luff up to cause windward to luff</b>, leeward may only come up to her proper course or tack and promptly sail astern of windward. If leeward can point higher, and that is her proper course, causing the windward boat to luff and/or tack, so be it.
There are good explainations w/illustrations in the Team Sailing Case Book, Calls A1, C4, D5, D7, F1, F3, F4. Pay attention to the explaination of figure 2 in Call F1. It shows a simple way the proper course restriction can be broken.
http://www.sailing.org/rrs2005/trcallbook2005.pdf