The BoJ is announcing, yet again, that it intends to intervene to weaken the yen as need be.So far the attempts at yen intervention haven’t very much. Over short periods of time the BoJ can weaken the currency by printing and buying other currencies, but that hasn’t shown to change anything meaningfully.
So the idea seems to be, at least make traders terrified of bidding up the yen, by letting them know that they will intervene at any time and create big losses for them. It doesn’t change anything fundamental, but maybe it could take out any speculative froth buoying the currency.
Here’s a look at dollar/yen since before the first intervention. You can see how it’s almost all eroded.