If I spend time and resources on this line of action, X will happen which will help me achieve Y.

It simply means you’ve thought through how you will achieve an objective (Y).

It also includes contingency for emergent situations — for example what if X does not happen; and how can I make sure I’m still better off if I do not achieve Y?

You start by figuring out your X and Y. Y is the desired objective — the final destination. X is the ‘mid point’ that causes Y.

Then you figure out what you must spend time and resources on for X to happen. That’s your line of action.