What is [Use FX1 and FX2 as FX SEND/RETURN] of [Preferences] window > [Controller] tab > [Effect] tab > [BEAT FX] in PERFORMANCE mode?

This is shown only when an FX SEND/RETURN compatible DJ equipment*1 is connected and selected as the audio device.

When you tick this option, FX (e.g. ECHO or REVERB) tails can be heard even after pulling the channel fader of the mixer all the way down. Please note that rekordbox dj FX1 and FX2 must be set to the same mixer channel in this case. This is because both of the FX1 and FX2 are routed to [FX SEND/FX RETURN] audio routing of the mixer.

When you untick this option, the FX1 and FX2 are independently routed to the internal audio routing of rekordbox dj. Therefore you can set the FX1 and FX2 to separate mixer channels. However, FX tails can NOT be heard even after pulling the channel fader of the mixer all the way down.