What does it mean that capture decisions and their reasoning?

Record not just what you decided but why — because your future self will rewrite the reasoning after the fact, and you will never notice it happening.

Example: You choose to turn down a promising job offer because you value stability for your family right now. Six months later, the company you stayed at announces layoffs. You tell yourself you 'always had a bad feeling about staying.' But your decision journal says otherwise — you stayed deliberately, for specific reasons, under specific constraints. Without that record, you would have rewritten your own decision history and learned nothing from a choice that was actually well-reasoned.

Try this: Pick one decision you made in the past week — it doesn't have to be big. Write down: (1) what you decided, (2) the 2-3 reasons that drove the decision, (3) what you expected to happen, and (4) what alternatives you rejected and why. Time yourself. This should take under 5 minutes. If it takes longer, you're over-thinking the format and under-practicing the habit.