Liberty from coaction is freedom from external compulsion contrary to your own will. E.g., when your older brother seizes your wrist and makes your hand slap your face and demands "Why are you hitting yourself" the slap was due to a necessity of coaction.
One can act under a natural or internal necessity, and still act freely. See Richard Muller, Dictionary of Latin & Greek Theological Terms, s.v., libertas a coactione; necessitas coactionis; liberum arbitrium.