Relax! No, really.
Terms like "dead relaxation" and "living relaxation" might provide a clue to what's going on. Then there's "relax completely". I think something like, "relax completely in all directions" is more helpful.
Just as a relaxation starting point, here's something I learned to see if someone can shut down all muscle activity to a particular body part. Ask your partner to let you
lift their arm from their side, up to their front, to a position where their hand is level to their shoulder. You can lift from their fingers or wrist. Tell your partner, that when you let go, their arm should fall naturally to their side. It is amazing how often I see their arm hesitate, upon release, before falling. Or how often I'll feel my partner assist in lifting their arm, even when they know
and have been told not to.
Of course, this is used primarily to illustrate the point of what one kind of relaxation can be. And how something so simple, can be difficult for someone to grasp. This can also be used as a first step to then acquiring much the same feeling in the arm, while keeping it raised.