The option essentially lets you try a permadeath run of the full game, which will end if main character Basim dies or commits any illegal actions (ie. killing civilians) that would normally see you kicked out of the Animus.
Full Synchronisation Mode will become available when you install today's patch 1.0.7 for Mirage, which launches at around 1pm UK time.
Once enabled, your permadeath run cannot be disabled. Cross-progression and cloud saves will become unavailable. When you do die (because of course you will), you'll get a stats screen showing your gameplay time, number of fights, cause of death, enemies kil
via Ubisoft