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When Tokyo's equity market is open, USDJPY and the Nikkei 225 futures tend to move together. A weaker yen helps exporters, so the stock index rises as the pair does. A sudden fall in equities often brings yen buying as a safe-haven move.

Scalpers sometimes watch the index as a leading indicator. The relationship isn't tidy. Sometimes the pair moves first, sometimes the index does, and the lag varies from seconds to minutes.

What's reasonably consistent is the reaction to a sharp equity drop. Risk-off flows push the yen higher, particularly on the crosses such as GBPJPY and AUDJPY, which can fall much faster than USDJPY itself.

The correlation weakens when the main drivers shift. A day dominated by US rates or Bank of Japan commentary doesn't link the two in the same way.

Practically, a small second chart of the index or an index future gives a sense of whether a yen move is part of a broader risk move or a currency-specific one.

Plotting the two on one chart with different scales, for a handful of Tokyo mornings, is enough to see whether the index leads, lags or simply moves alongside. The result tends to differ by month, which is worth knowing before relying on it.

Is the Nikkei a useful companion screen for yen pairs, or just clutter?

For me it's useful, but only as a filter rather than a trigger. I keep a small Nikkei futures chart under my USDJPY chart during Tokyo hours, and the thing I actually use it for is telling whether a yen move is part of a wider risk move. If USDJPY drops 15 pips and the index is flat, I treat it as currency specific and usually fade it smaller. If both are falling together, I stay out of any long.

I tried using it as a leading signal for a couple of months and logged every time the index moved a clear amount first. It led maybe a bit more than half the time, but the lag was all over the place, from a few seconds to several minutes, which made it useless for timing entries on a scalp.

The crosses are where it pays off most. On a sharp equity drop, AUDJPY has fallen two or three times as fast as USDJPY for me, so the index screen keeps me out of long yen cross trades on those mornings.