This is a rule that comes into play in the Clarify stage, specifically when you are clarifying a next action that you have to do yourself.

The rule goes like this: "If the next action can be done in two minutes or less, do it now, even if it is not an urgent or high-priority task."

The rationale for this rule is that if doing a task will take less time than organizing it in your lists and tracking it timely, it is much more efficient to do it the first time it's in your hands.