For me, it's caring about means vs. ends.

Rationalizing: I'm thinking about how to justify doing something I'm already comfortable with; adhering to a plan or position I've already accepted. So I invent stories about how the results will be acceptable, that I couldn't change them anyway, that if they are bad it will have been someone else's fault and not mine, etc.

Not rationalizing: I'm thinking about the ends I'm trying to accomplish. I'm not committed to an existing plan or position. I'm curious about what plan will achieve the desired results. If the results turn out bad, I'll critique my own plan and change it (I suppose we call that lightness) rather than blaming someone else.