Empress Effects Compressor - Manual (Page 7)

Sidechain
The sidechain connector on the compressor is a loop that lets you
alter the signal going to the gain reduction control circuit. It accepts
a ¼” TRS plug: Tip = send, Ring = return, Sleeve= ground.
When using the compressor on a bass guitar, it can be very useful to
add a high pass filter to the sidechain in order to avoid triggering
the compressor with high energy notes in the lowest octave.
Using the Meter
The compressor has a 10 segment LED meter that shows input gain,
gain reduction or both.
Red LEDs indicate gain reduction. Gain reduction values are marked
over each LED. Green LEDs indicate input volume. When “both” is
selected, overlapping gain reduction and input LEDs will appear
yellow.