Capón means castrated in Spanish and that’s basically what it means when applied to agave. The agave life cycl is simple, they sprout, mature over years – potentially decades depending on the variety – flower so that they can reproduce, and die.
As agaves are ready to flower they’ll form a bump at the top which is a sign that the quiote is about to start growing up into a flowering stalk. If you let the quiote grow it will pull up an enormous amount of the plant’s carbs and energy. The entire idea of castrating an agave is to keep all of those carbs in the piña of the agave so mezcal and pulque makers cut it off.
Pulque makers do this by cutting into the top of an agave when it’s fully mature and carving out a bowl there. This puts the agave’s defense mechanism into action and it starts secreting sap. Pulque makers scoop up that sap daily and ferment it into pulque. The agave keeps producing sap until it dies.
Mezcal makers will do something very similar: They wait for the quiote to start emerging from the top of the agave and cut it off as close as possible to the piña, or heart of the agave. They’ll wait for a few months until they feel that the agave is at its ripest and then harvest it. Mezcaleros claim that this results in a richer mezcal after distilling.
It’s a technique that is widely used as a way to make agaves which would otherwise start flowering at different rates across a field all be ready for harvest simultaneously. Sometimes a farmer will capón an entire field of agave for a specific mezcal. Recently some mezcals call this out explicitly like Noble Coyote and Rezpiral.
It’s important to note that preventing agave from reproducing sexually through their flowers causes great damage to them as a species. If we don’t allow agaves to reproduce sexually their genetic diversity will be reduced and the surrounding ecosystem will be destroyed because bats, hummingbirds, and many insects depend on agave pollen to survive. That’s why there’s a big move afoot to encourage agave farmers to let a portion of all their fields flower. The farmer loses a few agaves in the process but the ecosystem is maintained.
As a footnote: I know of one mezcalero who makes mezcal from the quiote which only goes to show that there are few absolutes in the story of Mexican distilling and that distillers are among the most creative people on the planet.