This will increase the audio gain matching the selected frequency when on the plus side. People voice got a broad range our ears are not linear and you may not notice a difference.
Let say you try to better understand someone under static noise by tuning the low range voice 300-600 Hz this will sound better because the 1000-2300 Hz will stay at the same level.
On CW we use it to match the programmed pitch frequency 600-700 Hz or else and we can reduce the volume control for less ear fatigue. I better like the APF for CW.
For RTTY I will select my programmed tone mark to 2125 Hz, this helps under some qrm and strong fading with 250 hz bandwidth.
I must agree that function better sounded on FTdx5000. Of course Ftdx101 series unlike FTdx5000 uses SDR software including AGC and could explain some differences.
73, va2am, Rejean