-1.0 disables the override, so the timestep authored in the simulation settings will be used (which may or may not imply a fixed timestep). A value of 0 forces a variable timestep to be used. A +ve value is used to specify a fixed timestep.
Usage
ControlRig.Physics.FixedTimeStepOverride [value]
Set via the console (~) or in DefaultEngine.ini / DefaultGame.ini.
