Pioneer Dj DDJ-FLX10 4-Channel DJ Controller for rekordbox and Serato DJ Pro (Black) - Manual (Page 32)

Part names and functions
32
En
The deck you pressed the button only plays the Vocal Stem, and the other deck
mutes the Vocal Stem.
32 ACTIVE PART INST button
Press:
Turns muting of the Bass and Melody Stems on/off.
[SHIFT] + press twice:
You can duplicate a track loaded on a deck so it plays on the other deck from
exactly the same point.
Press the button twice while holding down the
[SHIFT] button on the deck
where you want the duplicate track to start playing.
The deck you pressed the button only plays the Bass and Melody Stems, and
the other deck mutes the Bass and Melody Stems.