Yen crosses are the standard vehicle for the carry-trade, because Japan's policy rate was the lowest in the developed world for decades. Buying a higher-yielding currency against the yen collects the differential through the swap-rate.
That makes them a sentiment gauge. AUD/JPY in particular tends to rise with equities and fall faster than equities when risk is cut, since both the risk currency and the funding currency move against the position at once.
Example: a trader long AUD/JPY at 98.50 earns roughly JPY 700 per night per standard-lot in swap. A single 3% drop to 95.55 costs JPY 295,000, wiping out over a year of carry.
Related: carry-trade, carry-unwind, guppy, funding-currency