applies, the boat that tacked in the zone "shall not cause a boat that has been on starboard tack
since entering the zone
to sail above close-hauled to avoid contact".
Let's say I tack from port to starboard in the zone, finishing my tack clear ahead of a boat that has been on starboard since entering the zone, and slower than that boat.
Suppose we're both close-hauled when I finish my tack, and I tacked close enough to the other boat that (because I'm slower) they must take some avoiding action, either bearing off behind me or heading up above close-hauled. Suppose they choose to head up above close-hauled.
Have I broken 18.3
? It seems like yes: They sailed above close-hauled, and I did cause it.
But I'm uncertain because they also did have the option to sail below me, so they didn't have to sail above close-hauled.