Your definition is more correct.

I'm close to some kind of mastery with cello, and broadly we tell students to play with zero tension.

This is useful to say (often they have way too much tension and need to really dial it back), but in reality there is _some_ tension in everything:

- left hand: the fingers are basically a conduit for your back weight, but they need enough strength to stand up and _act_ as a conduit, otherwise they'd collapse. (but they needn't do more)

- right hand: weight flows from the back, down the arm, into the index finger, and all power derives from that + bow speed + how close you are to the bridge. However, the thumb needs to engage enough to counterbalance the weight on top of the stick, otherwise the bow would clumsily fall over.

The key is, as you say, doing the bare minimum.