The flow, in a technical sense, is what Matt said.
I also think of it as not simply in the sense of your flow but the flow of the "fight". You do not simply perform well, but perform well in accordance with what the opponent is trying to do to you.
Flowing with the situation, it's rythym, in other words. That's not very unique to FMA however, and my eskrima instructor points that out to me now and then.
"Success is a process, not a destination. Have faith in your ability."~Bruce Lee