The Fed may respond to a recession by

One answer is Lowering Interest Rates

The Fed may lower interest rates, which theoretically should (and usually does) increase the amount of money in circulation in the economy.

This is because banks are able to less expensively borrow money from the Fed. They also earn less on bonds from the Fed that they're holding, motivating them to sell them back to the government and invest their money elsewhere.

More money in circulation ideally stimulates economic activity.