Traktor’s Sync is based around a Master and Slave concept. The tempo of the Master deck dictates the tempo of all other decks that have Sync activated.
Not only is the tempo synced, but the transport is also synced meaning once you press play, Traktor will adjust the start point so the decks are always in time.
Load a track into Deck B and press play. Traktor will automatically assign it as the Master. Press Sync on Deck A and you’ll see the tempo fader is greyed out. This deck’s tempo is now slaved to the Master deck - Deck B.
Pro tip: If you like the idea of using sync to align the tempos of two decks, but don’t want the transport to be synced or quantised, there is a MIDI command called ‘Tempo Sync’ which can be custom mapped to a controller button or QWERTY key. This will match the tempos of each deck without turning on the sync function.