Using the equation of exchange (MV = PT), a collapse in velocity—people hoarding money because they're anxious—is a real problem during a downturn, but it is hard to measure precisely, so offsetting it with the money supply (M) is inherently difficult to do in a timely manner and the right size; this is why Friedman argued central banks should simply target a slow steady monetary growth rate (~3%) rather than try to fine-tune.