Pin the price of a bond and you give up control of how many you own. Japan pinned the 10-year for seven years and ended up holding more than half the market.
In September 2016, the Bank of Japan did something no major central bank had tried in the modern era. Instead of announcing how many bonds it would buy, it announced what price it would pay — pinning the 10-year government bond yield at around 0%, with a tolerance band of about 0.1% either side, and promising to buy whatever quantity was needed to keep it there.
— Educational, not financial advice.