Japan's new interpretation of "excessive" yen volatility is aimed at keeping investors on guard rather than lowering the threshold for actual intervention, say experts, who expect yen bears to remain resilient until domestic monetary conditions tighten.

Japanese authorities have traditionally defined excessive moves as an abrupt spike or plunge in the yen that happen in a short period of time, such as in a single day or a week, and driven by speculative traders.

But top currency diplomat Masato Kanda said on Wednesday that steady yen falls over a protracted period could also warrant stepping into the market, suggesting that Tokyo was giving itself wider scope to intervene to prop up the currency.