No luffing room from clear astern
The situation
On a reaching leg, Blue comes up from clear astern and establishes a leeward overlap on Green, less than two hull lengths away.
The question
Can Blue, now the leeward boat, luff Green up?
The ruling
No. Under rule 17, because Blue got the overlap from clear astern within two hull lengths to leeward, she may not sail above her proper course while the overlap continues within that range. She does have right of way as the leeward boat (rule 11) and Green must keep clear — but Blue can't use that to drive Green above her proper course.
A leeward overlap gained from clear astern within two lengths can't be used to luff above proper course (rule 17).
Opens the situation on the boat-length grid — scrub it and see exactly how the boats meet. Free, no account needed.
Rules cited
See the underlying rule: Rule 17 — On the Same Tack; Proper Course.