Inside overlap at the leeward mark
The situation
Two boats run down to the leeward mark on the same tack. As the first of them reaches the three-length zone, Green is the inside overlapped boat (closer to the mark) and Blue is outside.
The question
What must Blue give Green, and what if the overlap later breaks?
The ruling
Blue must give Green mark-room (rule 18.2(a)): room to sail to the mark and then to round or pass it on the required side. The overlap is locked in the moment the first boat reaches the zone — even if it later breaks, Blue still owes the mark-room. Green, for her part, must not take more room than her mark-room allows and must keep to the other rules while inside.
Inside overlap at the zone means mark-room, decided when the first boat reaches the zone — and it isn't lost if the overlap breaks.
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 18 — Mark-Room.