The pre-emphasis control acts like a side chain filter which is set flat (bypassed) when fully clockwise.
When turned anticlockwise it progressively enables a side-chain low pass filter so that the high frequency signals control the compression much more than the low frequency signals. This is how the original vintage limiting amplifiers work.
As the trimmer is turned counterclockwise, the compressor responds more to high frequency transients (Or alternatively as the trimmer is turned counterclockwise, the compressor responds less to low frequency transients.)
The pre-emphasis filter itself is actually affecting the side-chain low frequencies. When using the pre-emphasis filter, high frequencies are compressed more than low frequencies of a similar level. And yet the frequency response of the actual audio path remains flat any given moment in time.