Increasing the TK will decrease the volume for very soft sounds (counterpart to the Target 2.0 20dB tab). TK stands for Compression Threshold Kneepoint. If you change the view on iPFG to Input/output and then Target and select 500Hz for example, by increasing the TK you decrease the output for soft sounds. By decreasing the output, you decrease the amount of volume for those soft sounds.
HTH