Every dramatic currency move soon turns into a whodunnit, even when it is hard to pinpoint the perpetrator in a $5tn-a-day market. Algorithmic programmes are often blamed, along with fat-fingered traders. But the current mystery has a more official suspect with talk about whether or not the People’s Bank of China intervened last week to give the renminbi its biggest gain on record. But the better question to ask when the PBoC is involved is, what counts as intervention?

        每一次剧烈的汇率波动都会很快演变成一部悬疑片，尽管在一个日交易额5万亿美元的市场上很难确定谁是真凶。算法程序、还有按错键的交易员经常被当作罪魁祸首。但目前上演的这部剧有个官方的“嫌疑人”：人们在谈论上周中国央行是否出手干预，导致人民币汇率出现创纪录涨幅。然而，在说到中国央行时，更应该问的是，怎样才算干预？

        Old-school action, perhaps best typified by the Bank of Japan in the 1990s and 2000s, involved spending billions in mere minutes to reverse a market move. Call it the King Canute approach. See also the Bank of England in 1992. In the past decade, intervention has become more subtle with central banks trading to reduce the scale of unwelcome developments rather than to hold back the tide entirely. Asian central banks are experts in this, notably in Indonesia, South Korea and Malaysia, all of whom have been in the market in the past two months.

        老式的做法是在短短几分钟内，耗费数十亿资金来扭转市场走向——最典型的也许是日本央行(BoJ)在上世纪90年代和本世纪头十年期间的做法。可以称之为克努特王(King Canute)策略。1992年英国央行(BoE)也采取了这种路数。过去10年来，干预手法已变得更微妙，各央行通过交易来削弱不利走势，而不是完全阻止市场大势。亚洲央行精于此道，尤其是印尼、韩国和马来西亚的央行。过去两个月，这些国家的央行都进入了市场。

        China, however, is less predictable. The near $1tn fall in its currency reserves since July 2014 implies heavy action, but exactly how much, and whether on or offshore is unclear. PBoC officials rarely comment on anything, but they never confirm intervention, however likely it looks. This leaves traders to guess whether the flows they see are the PBoC spending its own dollars, Chinese banks trading for their own book or perhaps the PBoC asking those banks to make a splash with their dealing — which can affect the market mood, but is not technically intervention.

        然而，中国更加不可预测。自2014年7月以来，中国外汇储备减少近1万亿美元，暗示着中国进行了大规模干预，但干预规模有多大、是在岸干预还是离岸干预，都是未知数。中国央行官员很少就什么事情发表评论，更从不证实是否进行了干预，无论看上去有多像。交易员只得猜测，他们看到的资金流动究竟是中国央行在动用自己的美元储备、中资银行自己在进行交易，还是中国央行要求这些银行进行交易以制造轰动效应——这可以影响市场情绪，但从技术上来说并非干预。

        Perhaps the PBoC is being more subtle still: there is a strong 50-odd trading day pattern in onshore renminbi between a notable weak point — after which it rallies — and its next bout of weakness. This applies since China’s August 2015 devaluation. It could be that three calendar months is simply the time the PBoC judges the market needs to calm down, but it is also the length of many currency options. Could the PBoC be running out the clock on speculators? It has long had an aversion to traders profiting from the renminbi’s direction and this is one way of not making renminbi trades worth it — while allowing it to avoid the intervention questions.

        也许中国央行现在的手法还要更加高深：在岸人民币汇率从一个显著低点——之后会回升——到下一轮走低之间通常有50来个交易日。自2015年8月中国让人民币贬值以来就是这种情形。也许日历表上的三个月恰好是中国央行判定市场平静下来所需的时间，也是许多外汇期权的期限。难道中国央行在拖时间，让投机者在最后一刻什么都做不了？长期以来，中国央行一直不愿看到交易员通过押注人民币走向获利。这是一种办法，既不让人民币交易有利可图，同时又可以规避干预问题。