what's the opposite of hysteresis?

And did it happen in America in 1945-1973?

More Paleo-Keynesianism (Slightly Wonkish) by Krugman

This matters, a lot. The belief that the economy fluctuates around potential output, that it can’t be persistently below potential, is based ultimately on the natural rate hypothesis, which in turn took over economic thinking during the era of stagflation. This belief, in turn, underlies official estimates of potential output, which as Simon Wren-Lewis notes, causes any sustained slump to get built into official estimates of potential. Hence the official EU view that Spain is near full employment, the notion that Britain in 2007 was a hugely overheated economy with a huge structural budget deficit, and so on. If stagflation-era macro is wrong, so are all of these conclusions.