At least once a race you hear a skipper informing another skipper that that he is sailing above his proper course, and should adjust his course. If both boats are overlapped, within the two boat lengths laterally, and the leeward boat established an overlap from clear astern, then the statement is true. The leeward boat may not sail above his proper course, and adjust course while he is overlapped while within the two boat lengths.
As with any story there is always another part to it. If the leeward boat is sailing above his proper course, and did not effect the course of the windward boat, and the red boat calls a penalty, it may not stick (see example). The blue boat's course was a course that could reasonably be described as 'a course she would sail to finish as soon as possible in the absence of the windward boat' (definition of Proper Course and ISAF Case 25).
If the leeward boat needs to alter course so there won't be a problem of sailing above his proper course, the leeward boat must gybe, (see rule 18.4). If the blue leeward boat sails above his proper course without the right to do, the windward boat must still try to keep clear of the leeward boat, (see rule 11).
This all comes down to the statement, 'a course she would sail to finish as soon as possible in the absence of the windward boat'. Next time we'll go over this term of proper course, and see what it really means.