Section C rules do not apply at a starting mark surrounded by navigable water or at its anchor line from the time boats are approaching them to start until they have passed them. When rule 20 applies, rules 18 and 19 do not.
18.3 Tacking When Approaching a Mark
If two boats were approaching a mark on opposite tacks and one of them changes tack, and as a result is subject to rule 13 in the zone when the other is fetching the mark, rule 18.2 does not thereafter apply. The boat that changed tack
(a) shall not cause the other boat to sail above close-hauled to avoid her or prevent the other boat from passing the mark on the required side, and
(b) shall give mark-room if the other boat becomes overlapped inside her.
In each case notice that the yellow boat passes head to wind inside the zone and this triggers rule 18.3 because yellow "changes tack, and as a result is subject to rule 13" while blue is fetching the mark.