Rule 16 - CHANGING COURSE
16.1 When a right-of-way boat changes course, she shall give the other boat room to keep clear.
16.2 In addition, when after the starting signal a port-tack boat is keeping clear by sailing to pass astern of a starboard-tack boat, the starboard-tack boat shall not change course if as a result the port-tack boat would immediately need to change course to continue keeping clear.
There's no obligation on the keep clear boat to anticipate a course change by the right-of-way boat.
A boat keeps clear of a right-of-way boat
(a) if the right-of-way boat can sail her course with no need to take avoiding action and,
(b) when the boats are overlapped, if the right-of-way boat can also change course in both directions without immediately making contact.
The space a boat needs in the existing conditions, including space to comply with her obligations under the rules of Part 2 and rule 31, while maneuvering promptly in a seamanlike way.
Tack, Starboard or Port
A boat is on the tack, starboard or port, corresponding to her windward side.
Blue is trying to duck Yellow...
Yellow and Blue are on a collision course. Yellow is on Starboard, with right-of-way under Rule 10.
Blue has changed course, to keep clear by passing astern of Yellow. Yellow then changes course as well.
Blue continues to change course to keep clear, but Yellow also keeps changing course, until it is no longer possible for Blue to keep clear.
Yellow has broken Rule 16.
Last Second Course Change
Blue is keeping clear of Yellow, until...
Yellow and Blue are not on a collision course. Yellow is on Starboard, with right-of-way under Rule 10. Blue's course is clear of Yellow's course. If Yellow changes course here, Blue has room to also change course to keep clear.
If Yellow changes course here, Blue still has room to also change course to keep clear.
Yellow changes course, but Blue is not able to keep clear. Blue does break Rule 10, but will be exonerated, as Yellow did not provide room to keep clear under Rule 16.