For those who don't know tools, a ratchet is a device which allows something to move in one direction but not the other. A ratcheting screwdriver, for example, will only turn the screw in one direction, so you can twist it back and forth without having to let go.
When a technology is introduced that expands the capacity to produce food, population grows to use all that extra capacity. People don't generally voluntarily reduce population size, so the adoption of that technology becomes irreversible. (I would like to thank Garrett Hardin for introducing me the ratchet effect. It really is just an extension of Thomas Malthus's ideas and is central to Jevon's Paradox, that increasing efficiency in using a resource increases the overall use of that resource. Hardin was specifically concerned with food production, but the ratchet effect applies to many other endeavors.)
Just because people don't choose to do something, however, doesn't mean it doesn't happen. Technologies can be lost and populations reduced without any intention. When they cut down the last tree on Easter Island, being a lumberjack was obsolete. The bubonic plague significantly reduced the number of Europeans.
Going back to the tool analogy, a screwdriver which only tightens or only loosens screws isn't very useful. That's why ratcheting screwdrivers have a switch: flipped one way it tightens, flipped the other it loosens.
There also is an reverse ratchet effect. Extinction is a 100% phenomenon; a species is not extinct until every male or every female of a species is dead or incapable of reproducing. So too is it with technology. As long as someone somewhere in the world knows how to do something, the technology is not completely lost. With the key technologies that allowed populations to expand, this leads to a ratchet effect on the downside. Those who still have those technologies will have an advantage over those who don't, and they will grow in proportion to those who don't. (Please note I am talking about relative percentages, so if one group loses 50% of its population and the other loses 75%, the first has doubled in relative proportion to the second.)
No matter how bad things get in the short term, the reverse ratchet effect will determine where we resume the Long Ascent from.