Quote from illiquid:
I believe it was the Ministry of Finance selling bonds to the Bank of Japan, with the requirement that they eventually had to buy them back, so it's more like a loan for more firepower to sell yen since the BoJ is already near its yearly limit of yen sales. Any increase in this limit won't take place until the start of new fiscal year in April, so this is just a way of getting around it -- so not the same as selling treasuries on the open market.