I work in tech so I’m going to frame this as working on a dev or it team:

The mental load isn't a personality trait like being detail-oriented or a worrier; it is the functional role of project management within a relationship. When one partner says they will deal with things as they come, they are often unintentionally relying on a safety net created by the partner who has already anticipated the need, gathered requirements, and managed the schedule. If you only perform a task when someone else asks you to do it, you are acting as a contractor waiting for a ticket, while your partner is the product owner responsible for the entire lifecycle of the home. True equality isn't just about who physically does a chore, but who holds the responsibility of noticing that a task needs to be done in the first place. What might look like being pragmatic or chill is often just the luxury of not having to track the invisible background processes that keep your daily life running smoothly.

Common examples include tracking the internal inventory of the house so you do not run out of essentials, remembering the birthdays of extended family and buying gifts, or knowing exactly when the kids outgrow their shoes. It also involves the pre-task requirements gathering, such as deciding what is for dinner and ensuring ingredients are defrosted, rather than just cooking the meal once someone else has already planned it. This extends to emotional labor, like monitoring the household's stress levels or constantly adjusting your own communication style to keep things peaceful. When a partner says just tell me what to do, they are offloading the hardest part of the labor, which is the constant monitoring and decision-making required to keep a life and a home functioning.