Pressing Sync will match the both the BPM values and transients of your tracks together. If you have dropped your next track in but it’s not quite in time, by pressing sync you can perform an automatic beat sync.
Beat sync works by snapping the two closest transients together and matching the BPM.

You can click the OFF button to the right of the Sync panel to turn this off. This will cause the pitch to revert to the tracks absolute pitch as reflected on your hardware pitch slider controls.

When you engage Sync on a deck, that deck becomes the Secondary Deck while the deck it has been synced to becomes the Primary Deck. The synced BPM value will be taken from the Primary Deck.

Relative Pitch & Pitch Control
Pitch slider/dial adjustments on hardware of a deck in Sync will alter the tempo for all tracks in Sync.

When a deck is in sync, altering the pitch will affect the pitch of all synced decks. This is known as relative pitch.

Disengaging sync on a deck causes the pitch to return to its absolute pitch level as represented on the hardware slider.

Average BPM and Changing Tempos

Analyzing a file will only calculate the average BPM and there is a chance the track’s actual tempo may differ from the displayed BPM value. This will be the case for tracks with varying tempos (ie. some types of music such as rock, reggae, funk, jazz or live recordings), or the BPM may vary over time (in the case of vinyl recordings, old live recordings to tape, or poorly encoded music).