The defaultSolverIterations determines how accurately Rigidbody joints and collision contacts are resolved. (default 6). Must be positive.
If you are having trouble with connected Rigidbodies oscillating and behaving erratically setting
a higher solver iteration count may improve their stability (but is slower).
This value is usually changed in
Edit->Project Settings->Physics inspector instead of from scripts.
Note: Changing the defaultSolverIterations does not affect already created Rigidbodies. To change an existing Rigidbody please use Rigidbody.solverIterations.
See Also: Physics.defaultSolverVelocityIterations.