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 isn't synchronized.

If you use pitch bend on a deck that isn't the MASTER, the button will switch from lit to blinking. To switch back to lit (start syncing beat positions), press the BEAT SYNC button twice to turn off and turn on Sync again.