Sure I can give a quick refresher.
Basically, different shards have different aetherial balance. An aetherial balance can be pushed to greater heights, which causes aether to flow back towards the source. Ascians attempt to manipulate events to cause this imbalance. In the case of the 13th, they sowed chaos and destruction through the faulty Auracite. On the first they had their mysterious plan with Ardbert and pals. When imbalanced, the shard can be Rejoined with the source by causing a calamity on the source.
However, if the aetherial balance tips too far, it can cause a Flood. A flood royally messes up a shard, usually leaving it an aether deprived wasteland and causing all kinds of other issues. When this happens, there's basically nothing left to rejoin. Causing a flood is never really the Ascian's plan. It can make a Shard useless. Still, they're far from perfect, and they basically messed up twice. If the 1st's Flood had completed, there wouldn't be a shard left to rejoin. That's probably why they rushed to have Ardbert cause a calamity, so they could rejoin the 1st before it was destroyed. However, Minfilia stopped the Flood, which left them an opening. They just had to tip the Aetheiral balance just right and then cause a calamity on the first. Something they would have been successful in if not for our heroes intervention.