Two port-tack boats are sailing close-hauled toward the windward mark, outside the zone (so Rule 18 does not yet apply).
A starboard-tack boat is also sailing close-hauled on the opposite tack (the “stand-on” boat under Rule 10).
The inner port-tack boat wants to tack because she is on a collision course with the starboard-tack boat.
However, the inner boat only shouts “woi tack!” — and does not use the proper call “Room to tack for an obstruction” as required by Rule 20.1(a).
It's safety rule, what shall starboard boat do?
Bear in mind that, in this scenario, PL is exercising a choice under RRS 19.2(a): she can always change her mind and bear away and pass astern of S, and accept her obligation to give PW room to pass between her and S under RRS 19.2(b)