Central Bank Intervention in Forex Explained

The Desk’s Guide

By Ken Chigbo, Founder, KenMacro, 18+ years across discretionary and systematic strategies, UK macro desk.

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Updated 2026-05-23

What FX intervention actually is

A central bank intervenes in the foreign exchange market when it decides the exchange rate has moved too far, too fast, in a way that damages the domestic economy. The mechanism is straightforward: if the currency is weakening sharply, the central bank sells foreign currency reserves and buys the domestic currency, adding demand to the spot market. If the currency is strengthening to a point that hurts exports, the central bank sells its own currency and accumulates foreign reserves. This is not a policy rate decision. It sits alongside interest rate policy, and the two can conflict. Japan made exactly that point in 2022 when the Ministry of Finance bought yen to defend 145 while the Bank of Japan was simultaneously running ultra-loose monetary policy to suppress yields. The intervention bought time, but the contradiction between the two stances was visible to any macro trader watching both markets at once.

Verbal intervention versus actual flows

Before a central bank spends reserves, it tends to talk first. Officials give interviews, speeches or press conference comments warning that the current rate is disorderly, unjustified by fundamentals, or being closely monitored. This is jawboning, and it works precisely because markets know real flows could follow. The Swiss National Bank used this playbook from 2011 onwards, repeatedly reaffirming its commitment to the 1.20 EURCHF floor even as the cost of defending it grew. The key distinction the desk makes is credibility: a central bank with large reserves and a track record of following through on warnings moves markets on words alone. A central bank known to bluff does not. Watch the follow-through. If officials warn three times and deploy no real flows, the fourth warning gets discounted. If they deploy within hours of a statement, the next statement carries full weight immediately.

The BoJ yen defence and the SNB floor removal

The two most instructive modern case studies are Japan and Switzerland. Japan conducted its first yen-buying intervention since 1998 on 22 September 2022, with the Ministry of Finance deploying roughly 2.8 trillion yen in a single day. The dollar fell more than seven yen intraday. Additional interventions followed in October 2022, and the total yen-buying across 2022 reached around 9.1 trillion yen. By 2024 the yen had weakened to a 34-year low, and Japan confirmed a further 9.79 trillion yen in intervention between late April and late May 2024, taking the 2024 full-year total to approximately 15.2 trillion yen. The SNB story runs in the opposite direction. In September 2011 the SNB set a floor of 1.20 on EURCHF, pledging to buy euros in unlimited quantities to prevent the franc from appreciating beyond that level. For more than three years the floor held and volatility in EURCHF collapsed. On 15 January 2015, with no prior warning, the SNB abandoned the floor and cut its deposit rate further to negative territory. EURCHF fell from 1.20 to as low as 0.85 on some platforms in the first minutes of trading, a move of roughly 30 per cent in one of the most stable pairs in forex. The lesson from both episodes: intervention creates artificial equilibria that produce violent mean-reversion when they break.

Sterilised versus unsterilised intervention and why reserves matter

When a central bank intervenes without adjusting its domestic monetary operations to compensate, the intervention is unsterilised. Buying domestic currency pulls liquidity out of the financial system, which effectively tightens monetary conditions. When the central bank offsets that effect by buying domestic bonds, or conducting repo operations to inject liquidity back in, the intervention is sterilised. The rate may still move, because the raw FX flow has an impact, but the domestic money supply and short-term interest rates are left unchanged. Sterilised intervention is widely considered less durable because it does not alter the underlying interest rate differential that was driving the currency move in the first place. What constrains every central bank regardless of the sterilisation choice is the size of reserves. A country defending a weak currency must sell foreign currency reserves, and those are finite. The pace of reserve depletion is the most direct indicator of how long a defence can be sustained. Traders track monthly reserve data from central banks and the IMF for exactly this reason.

How the desk trades around suspected intervention zones

Intervention is most likely at round numbers or at levels that officials have publicly referenced as disorderly. On USDJPY, 145, 150 and 155 have all functioned as informal trigger zones in recent cycles. The desk does not fade the intervention in the direction it is running. Stepping in front of a central bank with reserves is a fast way to lose. Instead, the playbook is to wait for the initial move to complete, measure the distance from the pre-intervention print to the post-intervention level, and watch whether the rate drifts back toward the trigger zone over the following sessions. If it does, a second round is likely. If price consolidates at the new level and verbal warnings resume, the intervention held and the thesis is intact. For confirmed intervention days the desk widens its volatility assumptions significantly. Spreads expand, slippage increases, and quick moves in either direction are possible as the market guesses the size of the programme. The practical implication for live trading is to check the economic calendar for any scheduled central bank comments, and to size down on pairs where intervention risk is elevated. Time-sensitive reserve and rate data are available at the economic calendar on kenmacro.com/economic-calendar/.

Frequently asked

Can a central bank always defend its currency?

No. A central bank defending a weak currency must sell foreign reserves, which are finite. Once reserves fall to a level the market judges inadequate, the defence collapses. The 1992 sterling crisis and the SNB floor removal in 2015 are both examples of a commitment the market decided to test to destruction.

What is the difference between a currency peg and an intervention floor?

A hard peg ties the exchange rate at a fixed level and usually requires the currency board or central bank to hold reserves equal to the entire monetary base. An intervention floor is a softer commitment where the central bank pledges to buy the currency if it weakens past a stated level, but the central bank retains discretion over when and how much to deploy.

Does verbal intervention alone move the market?

Yes, if the central bank has credibility. Markets price in the probability of real flows following the warning. A central bank with a track record of acting on its statements can generate a significant rate move with words before spending a single dollar of reserves.

Why did the SNB removal of the EURCHF floor cause such a large move?

The floor had suppressed volatility for over three years, which led many traders and businesses to hold large unhedged EURCHF positions on the assumption the floor would not break. When the SNB removed it without warning on 15 January 2015, the market had to unwind those positions immediately with no buyers at or near 1.20. The result was a fall to as low as 0.85 on some platforms before liquidity recovered.

How can traders tell whether intervention is actually happening?

The clearest real-time signals are a sudden sharp move in one direction at a round number or a level officials have flagged, unusual volume or spread widening in the early Tokyo or London session, and follow-up official statements confirming action. After the fact, monthly reserve data from the central bank and the IMF will show the drawdown. Japan publishes its intervention data with some lag via the Ministry of Finance.

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