What matters is how long USD/JPY stays down in the wake of suspected FX intervention that has seen the pair dive from 160.24 to 154.20 before rallying toward 156.

How far USD/JPY falls is less important than where the pair ends up, as an inability to hold the pair down will fuel greater demand.
Should USD/JPY exceed the level where BOJ was first suspected to have sold, a big rise could follow that the central bank cannot stop easily, and may fail to hold altogether without help.

The less time USD/JPY stays down, the greater the bullish reaction.
So once intervention begins it must continue, and perhaps grow in intensity if the central bank wishes to suppress USD/JPY at a lower level.

The influence of intervention when USD/JPY is high - as it was above 160 - is much greater than it will be once USD/JPY has dropped.

The bets against the yen will be reduced in the wake of this big drop.
The less wagered against the yen, the less the restraint on its drop.

While Japan's central bank maintains bond purchases, it's effectively fighting without itself for control of the yen.
Because bond buys are bigger, and are being sustained over a much longer period than intervention, their negative influence is greater, driving the yen to a record low in February this year.

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