As the yen slid past ¥145 to the dollar with barely a murmur from Japanese policymakers in recent days, suspicion grew that they won't be as quick to order intervention as they were last year as they now reap some benefits from a weaker currency.

Surging exports helped economic growth hit 6% on an annualized basis in the second quarter, and lower global oil prices have helped keep a lid on the import bill.

But a key factor behind the yen's weakness is unchanged — namely the yawning yield gap with the United States.