The default value 1.0 gives a non-damped Newton method. Common values are in the range 0.5 to 0.7 for somewhat conservative method and seldom become smaller than 0.2 for very conservative approach.
Additional info
From NumLib/ODESolver/NonlinearSolver.cpp line 426
This is a required parameter.
Data type: double
Expanded tag path: nonlinear_solvers.nonlinear_solver.damping