Investors take for granted that the Federal Reserve controls interest rates. Rarely do they have to think about how.

But a surprisingly lively couple of days in short-term money markets has meant that the “how” became nearly as important as the “why.”

The stress started on Monday in the market for repurchase agreements, or repos. The repo market channels more than $1 trillion in funds through Wall Street every day, usually without fanfare. That money is used to pay for the day-to-day operations of big banks and hedge funds.

Then the Fed’s key interest rate, known as the federal f unds r ate, hit 2.3 percent on Tuesday. That’s above the central bank’s target, and the rise reflected unexpected strains.