The reasons:
- Actual intervention has produced moves of several hundred pips in a short time, as in 2022 and 2024. No scalping stop survives that cleanly; the slippage alone can be several times the planned loss.
- Rumours and verbal warnings can cause sharp spikes with spreads blowing out.
- Technical levels stop working, because one very large participant isn't trading off the chart.
A worked example of the risk: a 0.5% risk trade with a 6 pip stop. A 200-pip move through the stop, even with only partial slippage, can turn that into a loss many times the plan. One such event can erase months of scalping.
Does anyone keep trading the yen in those weeks? If so, how do you size it?