CALL UMP 38          

Rule 12 On the Same Tack, Not Overlapped
Rule 21.3 Moving Astern
Definitions Clear Astern and Clear Ahead; Overlap

Question
In the pre-start, Blue and Yellow are both on the same tack with Yellow clear ahead. Both boats start moving astern by backing their sails and are still not overlapped.
Which boat has the right-of-way?

Answer
Rule 21.3 does not apply, because both boats are moving astern by backing a sail. According to the definition Blue is clear astern and therefore required by rule 12 to keep clear.
Whether one boat is clear astern of another or overlapped with her does not depend on the direction of motion of either boat.