I am still not getting it.

but a rigid body is a very crude model of a body.

it assumes all particles of the body are rigidly connected, but that assumption brakes down very quickly the moment a body is big.

bodies are elastic, and it is that property the let the lose energy when they collide, so on the extreme cases even a small rigid body is no completely accurate, and certainly for large bodies the force do not transfer instanstaneslly for one end to the other, oddly enough is it this property that make iteration solve perform more or less acceptable and is some case they work better than exact matrix solvers. But you can no count on that because that like say a broke clock can be right at lest twice a day, and assume that broke clock work. you can never be right for the wrong reasons.

For large bodies the inertia tensor is not longer constant and there are model the can simulate that by applying some linear and even quadratic stress.

these model still can use the rigid body model with some corrections, but the newton engine does not deal with that.

as for the question not there is not way the you can consider parts of the San to do body are movable.

in the future I will add the elastic model, but it will not use those stress model which are more appropriate to model vibrations, like building or large vehicle like ship and air plains.

instead I will use the stiff spring damper, on tretraheralized meshes, but for that we will use High Performance Computing.

that's for after newton 3.14, maybe newton 4.0