Here are a couple of diagrams to illustrate the purpose of rule 17. I've shown them with boats on a beam reach, but the principle is the same for boats on a beat to windward, or on a run downwind.



In this first example, Blue did not overtake Yellow from clear astern. Therefore rule 17 does not apply. Blue may sail above her proper course (provided she complies with rule 16.1) and take Yellow head-to-wind in order to prevent Yellow from passing her. Yellow must keep clear per rule 11. Neither boat breaks a rule. Yellow should lose this protest.



In the second example, Blue did establish overlap from clear astern within two of her hull lengths so rule 17 does apply. Blue may not sail above her proper course. Blue luffs Yellow up anyway (in order to pass through Yellow's wind shadow). Yellow must keep clear per rule 11 (and she does). Blue breaks rule 17. Yellow should win this protest.

I hope that helps,
Eric