Pioneer Dj DDJ-FLX4 Portable 2-Channel rekordbox DJ and Serato Controller (Graphite) - Manual (Page 21)

Part names and functions
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4 CUE/LOOP CALL  button
Press (during loop playback):
Halves the loop playback length.
Press:
Selects the loop slot.
5 CUE/LOOP CALL  button
Press (during loop playback):
Doubles the loop playback length.
Press:
Selects the loop slot.
6 BEAT SYNC button
Press:
The tempo (BPM) and beatgrid of multiple decks are automatically
synchronized.
[SHIFT] + press:
Switches the range of the [TEMPO] slider each time the button is pressed
while holding the [SHIFT] button.
Switch between [±8%]  [±16%]  [±50%]  [±8%] .
Press for at least a second:
Cancels Beat Sync.
7 TEMPO slider
Adjusts the playing speed of the track.
Serato DJ Pro
Serato DJ Pro