Official comment
When the slave track is synced to the master, the playback tempo is changed while the physical tempo slider remains stationary. The slave tracks then becomes the Master track and the tempo can be reset once the physical slider is moved to the position of the playback tempo. For example, if the track is being played back at +2.5%, you must move the tempo slider to the +2.5% mark and then you will regain manual control of the tempo. This behavior is the same as the sync feature on the CDJs.