i can't list all the lenses tha have this device to soften the image
they don't all do the same thing ...

i used to have a vitax and remember the know moved the rear element
i know some users re-focused for whatever reasons they had, after they turned the knob
i used it when i was stopped down beyond 4ish so i would still have the soft effect and not much DOF.

i know some folks modify their velo so it softens even more than designed, folks unscrew the front element of their favorite lens 1/4 turn
or remove the front element ( almost ) from their teleoptar ...

whatever it is that you might do, do you focus, defocus and refocus? do you focus, stop down a little and defocus paying attention to the ground glass,
do you shoot wide open and defocus again?

i got in the habit of focusing on something either infront of or behind my subject, if it wasn't a portrait ( or sometimes i do ) and gave up on the defocus knobs and unscrewing the lens &c

what is it that you do, and what sort of effect ( good or bad ) does it give? and what DON'T you do ?