The Bank of Japan buys a lot of government bonds, which is equivalent to the return of government bonds from the market to the central bank, while the yen flows from the central bank to the market.

First of all, buying a lot of government bonds will naturally push up the price of government bonds, and then the interest rate will fall (the price is inversely proportional to the interest rate). Falling interest rates will lead to a large amount of funds flowing out of the Japanese market in the foreign exchange market, that is, people used to hold yen, but now they are starting to exchange it for foreign currencies such as US dollars, euros and Australian dollars. As the supply of yen increases, it naturally depreciates, so the exchange rate decreases (referring to the direct exchange rate).

Secondly, the flow of yen from the central bank to the market has increased the supply of yen in the foreign exchange market, which will also lead to the depreciation of the yen.

So it is the result of lowering the exchange rate.