Empress Effects Phaser - Manual (Page 7)

Sensitivity
The sensitivity knob controls how the pedal listens to your input
signal when in auto mode. For softer input signals you should set
the sensitivity to a higher value. For louder input signals it should be
turned down.
In an auto mode that uses trigger detection, the red LED above the
tap switch indicates that a trigger has been detected. If you find the
auto mode is missing some of the triggers, increase the sensitivity. If
you find the auto mode is triggering when it shouldn’t be, decrease
the sensitivity.
When the pedal is in a submode that uses envelope detection the
red LED above the tap switch will light up when it is detecting
maximum volume. Ideally, in envelope based submodes, the LED
will only light up when you’re playing your absolute loudest. This
will ensure you are experiencing the full range of parameter sweep.
Setting the expression switch to ‘speed’ will allow you to control the
sensitivity with an expression pedal.