When the [BEAT SYNC] button is lit, both the beat position and BPM of the track are synchronized. When the LED is blinking, only BPM is synchronized and the beat position is not synchronized.

If you use pitch bend on a deck that is not the master, the button will switch from lit to blinking. To switch back to lit (and start syncing beat positions), press the [BEAT SYNC] button twice to turn Beat Sync off and on again.