Gold priced in yen moves on two separate sets of drivers at once: whatever is pushing the dollar price of gold, and whatever is happening to USD/JPY, including the Bank of Japan's own interventions in the currency market.
1. A cross built from two legs
XAU/JPY isn't quoted directly against a single, unified market the way XAU/USD is. It's effectively XAU/USD multiplied by USD/JPY. That means a quiet day for dollar gold can still be a volatile day for XAU/JPY if the yen is moving sharply on its own, and vice versa.
2. When the Bank of Japan steps in
When USD/JPY moves far enough to concern Japanese authorities, the Ministry of Finance and Bank of Japan have historically intervened directly in the currency market, buying yen to slow or reverse the move. These interventions tend to be sudden and large, producing sharp USD/JPY reversals within minutes that ripple straight through to the XAU/JPY cross.
The yen's long-standing role as a low-yield funding currency for carry trades adds another layer: BOJ policy shifts, such as ending yield curve control or raising rates, can trigger rapid unwinding of yen-funded positions across many markets at once.
3. Adjusting for the extra volatility
Because this cross carries volatility from two independent sources, position sizes calibrated to XAU/USD's typical range can be too large for XAU/JPY. Reducing size and widening stops to account for the yen leg's own swings, particularly around known BOJ policy meeting dates, helps keep risk consistent across instruments.