Automatic volume analog pedals.
Between instrument and an output to an amplifier.

UTILIZATION

Simpler than that, there is not.
1 Attack knob:
When the value is low, the effect is obtained with the strongest attacks.
When the value is raised, the effect is applied even on the lowest notes.
Knob one direction:
Time required for the volume reaches its maximum level from the time the note is played.

SOUND QUALITY

The effect is effective, but after playing the rope (string file or not), do not apply the same attack, nothing trs complicate quickly take the blow.
More effective directly out guitar rather than in the effects loop of the amp.

OVERALL OPINION

INTERESTED PDAL a condition to require this type of effect.
If not, go your way.