It is one for free, then you downgrade an action to take another one right?

correct. Or you can still take your action, suffer 2 strain, and take a second maneuver.

The most important thing to remember is that you can never take more than 2 maneuvers per round*.

Unless you have talents or special abilities that says otherwise.

Don't forget you can "buy" another manoeuvre for 2 Advantage instead of 2 strain. You still can only do two manoeuvres in a turn, though (unless you have the Unmatched Mobility signature ability).