In practical terms, there are at least three ways to make sure the Fill is less bright than the key:

1)You could use a scrim or screen to cut down the intensity of the fill light.

2) You could use a fill light with a lower brightness than the Key.

3) You could simply move the fill further away from the subject.

Again, look closely at the image to the right. It's very important that you notice that the brightness is NOT even across the face. One side is brighter. The result is that the face appears more three dimensional--and that's the point of this whole enterprise.