Comentário oficial
The reason a Master deck is used is because of more than 2 decks, plus the ability to have other hardware act as the master source (eg. a DJM, SP-16, DJS-1000).
There are two sync modes; a full sync and a BPM-only sync. Pressing SYNC the first time snaps the beats together and puts them in BPM lock. If you press it a second time, it disables the BEAT SYNC mode, keeping the tempo. If you want to fully disable it, you'll have to move the pitch fader to "pick up" the tempo.