Subsurface Color Value
While you can adjust the amount of Subsurface scattering that happens via the Opacity input, you can also adjust this using the Value slider in the Color Picker.
For example, setting the Opacity to a value of 1.0 and setting the Subsurface Colors Value from White to Black will effectively turn off Subsurface Scattering like in the example below.
If you do the opposite of this, adjusting the Value slider in the Color Picker so that it is now in the White, you will re-enable Subsurface Scattering like in the example below.
Here is an example of adjusting the value in real time. Notice how as the Value of the color is adjusted from Red to Black, the amount of Subsurface scattering influence is affected.