Simple Sync does a simple transient sync to the opposite track on the deck. You do not need Beat grids in order to use Simple Sync.
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 left of the Sync button to turn sync off (or press shift + SYNC on your Serato DJ hardware). When using a controller, one press of the OFF button will fully disengage Sync. However, when using Simple Sync with vinyl or CDJs, the first press will disengage Sync, but will keep the decks in Relative Pitch. This is shown by a blue outline around the SYNC button.
A second press of the OFF button 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 Slave Deck while the deck it has been synced to becomes the Master Deck. The synced BPM value will be taken from the Master Deck. The Sync button should appear blue and there will be no beat position indicator with Simple Sync.
TIP: You can use Simple Sync as a way to auto-tempo your tracks. Load a song, press Sync so the BPMs are the same and then drop the track in yourself. You can engage Simple Sync when using vinyl or CDJs in Relative or Internal modes.