Where to put DigiTech FreqOut in signal chain?
For best results the FreqOut should go at the front of your effects chain, though it would also probably be quite happy placed after a compressor or buffer. It's sometimes necessary to damp unwanted strings to avoid spurious sounds when sustaining a single note, just as it is when controlling real feedback.
How does the FreqOut pedal work?
The DigiTech FreqOut pedal uses pitch detection technology to constantly monitor the harmonic content of your guitar signal. If the guitar signal completely dies away, the feedback stops. Note that guitar signals persist on the electrical level longer than they are audible in some situations.