From Krugman's macroeconomic textbook (highlighting is mine):
"In an open-market operation the Federal Reserve buys or sells some of the exist- ing stock of U.S. Treasury bills, normally through a transaction with commercial banks—banks that mainly make business loans, as opposed to home loans. The Fed never buys U.S. Treasury bills directly from the federal government. There’s a good reason for this: when central banks lend directly to the government, they are in effect printing money to finance the budget deficit. As we’ll see later in the book, this can be a route to disastrous levels of inflation."
But how does this indirectness prevent the central bank from printing money to finance the budget deficit? If the government can order the central bank to buy its bonds, then it can also order the central bank to buy its bonds indirectly, during its open market operations. I fail to see how this indirectness of buying bonds serves as a safeguard against abuse of of money printing press by the government.