The "Spring Effect Strength" slider is a feature of LGS accessible only by force feedback gaming devices. This slider controls the strength of the spring forces within a game. Spring forces are the effect of the wheel/stick being pulled into a certain direction by the game/software. The farther that you move the handle away from the direction that the force is pulling in, the stronger the force effect will get. By moving the slider above 100%, the stronger that resistance will be. An example of this would be trying to pull out of a dive in a flight simulator. The more you try to pull out of a dive, the more resistance you will encounter.
The default centering spring that centers the joystick handle or steering wheel in games that do not support force feedback will not be affected by this slider. However, games that support force feedback may or may not disable the default centering spring.