They've been buying their own debt with their central bank, aggressively debasing their standard of living to try to relieve the extreme debt pressure & interest that has been consuming nearly half their tax revenue.

The central bank buys up the government's outstanding debt at zero interest, removing external holdings of debt, while punishing the Yen and anyone holding Yen assets in the process. For the government it becomes a low cost approach to cancelling out debt. For the people of Japan, it becomes a stealth inflation attack on their standard of living. The choices are slim though, they already have high taxes, and the national savings rate has dropped from high to nearly zero (formerly the people of Japan funded the big debt binge with the high savings rate).

It's the next level up from what the Fed was doing with QE. The Fed - supposedly - will sell a lot of its assets back into the market. The central bank of Japan plans to just buy up its own debt and cancel it perpetually. The Bank of Japan owns something like 43% of the Japanese Government's debt at this point.

You can almost guarantee the US Government & Fed will do the same thing in the next ~15 years, as US public debt hits $30+ trillion. If the US wanted to push its debt interest costs toward zero over time, it could have the Fed start buying up all the public debt. The cost would be debasement of the USD (the dollar would fall, commodities would soar, the US standard of living would fall, real inflation would spike). If you want a functioning market for your debt, you have to pay investor's rate of desired interest. You can massage that to some degree, which the Fed does, to try to keep interest costs under control. In Japan's case, they've gone full QE, entirely dropping the pretense of a market for their debt.