What are the ways to charge up the Synergy gauge?

You gain ONE Synergy Shard every 5 turns when a DBB unlocked unit is in your squad.
You gain ONE Synergy Shard when a DBB unlocked unit remains in Overdrive for one turn. (Up to 4 shards per Overdrive)
You gain ONE Synergy Shard each time a unit's UBB is activated. Note that even if the UBB is re-activated using a re-activation effect, only ONE Synergy Shard will be gained.
A maximum of TEN Synergy Shards can be held.