When you know your country (in this case Japan) is printing massive amounts of money and is debasing its currency, you can profit from this. You just borrow the debasing currency (yen) at ultra low yields and with that money you buy high yielding assets like the S&P500 or high yielding bonds in other countries.
So if the yen for example declines against the U.S. dollar (blue graph goes up), then you buy the S&P500 (red graph goes up). It also goes the other way round. When the yen starts increasing in value, people are going back into the yen to repay the borrowed currency by selling their high yielding assets.
Another example of carry trade is the U.S. debasing its currency with QE while emerging markets profit from this money printing. This carry trade is now unwinding as currency is flowing out of the emerging market currencies.
The key is: monitoring the USD/JPY exchange rate.