Line6 Catalyst What’s NOT in the Pilot’s Guide   Part 1 – The FX Loop The switch on the back panel serves only one purpose – to determine whether the signal on the FX Return (Power Amp In) passes through the on-board FX.   The Catalyst Global Settings FX Loop Switch determines whether or not the FX Loop is in the signal chain. OFF – internal signal BYPASSES the FX Loop completely. ON – internal signal goes OUT the SEND and/or external signal enters through