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Under yield curve control, a central bank targets a yield on a particular government bond maturity and commits to buying whatever quantity is required to hold it there. Quantity becomes the variable that adjusts, while the price is fixed within a stated tolerance band.

The currency link arises because the bank's bond buying defends a yield that the market may think is too low. When global yields climb, the gap between the controlled yield and its peers widens, and the domestic currency tends to weaken as capital seeks better returns. The policy is then tested, since defending the cap can require very large purchases.

Adjustments to the band are therefore high-impact events. A widening of the tolerance range, even if described as technical, can be read as a step toward policy normalisation and trigger a sharp currency rally. Markets look for tell-tale signs such as unscheduled bond purchase operations and unusually thin trading in the targeted bond.

For short-term traders, the key risk is discontinuity. A regime that suppresses volatility for long stretches can end abruptly, producing gaps rather than smooth moves, and quiet conditions beforehand may understate how fast repricing can occur.

Japan is the obvious case. When the BoJ held the 10-year JGB yield cap while global yields rose in 2022, the yield gap pushed USDJPY from around 115 to above 150 in a few months. The BoJ had to widen the band in December 2022, which caused a sharp yen rally that day.

For traders, the important point is that the end of YCC is usually announced suddenly and in steps. Each step moved the yen a lot. So positions that depend on the cap holding carry a hidden event risk around every policy meeting.