It's not just about defaulting on debt, more about using it to stimulate the economy.

https://www.brookings.edu/blog/up-front/2020/06/05/what-is-yield-curve-control/
This article is quite interesting in explaining how it actually reduced all the bonds they had to buy after implementation (until the current mass sell off)

There's a lot of game theory involved and now the tides are turned, this article is barely two years old and quite amusing everything is different now.

KingZing

bitcoin

10Y bond yields up 80% in Japan today. I guess the freefall started today.

taxd

I recently started reading about yield curve control (YCC), which is the strategy their central bank has taken. If you google "yield curve control" + "federal reserve" you get articles about Yellen, Powell and previous Fed hotshots all being "interested" in the idea. 

The whole idea (not a prof. econ guy, but I am a math guy) seems to be to stop government from defaulting on the debt by controlling the more future-oriented bond rates (10 year bonds), essentially trying to control or "peg" the interest rate to some value. It seems insane, you basically just keep printing money to throw at a problem to keep up appearances. 