How has Japan managed its debt ratio?

They have been using an implicit sovereign wealth fund, even in light of rising debt levels:

Since 2012, the social security fund has increased its exposure to riskier assets. In 2013, a government panel recommended a major reallocation of government-run pension funds into higher-yield investments (Hoshi and Yasuda 2015). Following this shift, three-quarters of public pension fund assets were allocated equally across three categories of risky assets: domestic equities, foreign equities, and foreign bonds (each at 25 percent). Over the past decade, these investments have generated strong realized returns, boosting the net asset position of public pension funds from around 40 percent of GDP to over 60 percent of GDP.

And this:

In fact, Japan’s net liabilities decreased over the past ten years from 118.4 percent to 77.6 percent.  However, these gains have come from taking risks — in particular, the risks of duration mismatch and currency mismatch.

That is from a new JEP article by Yili Chien, Wenxin Du, and Hanno Lustig.  Good work, but of course that makes Japan correspondingly vulnerable in the event of a major global equities downturn.