I did a few of them in the Beta. They're hard to time if you lag at all and they don't tell you what they do (that's normal though for FF games).
They're kind of neat, though, and easier to pull off except for the triggering. You queue up skills while you're stuck sitting around waiting for the queue to fill and the mob goes nuts on you and then you fire it off. It's the double tap that gets you... that and the game insisting that multiple people are trying to fire it off when there's only two of you talking on Teamspeak going "You hit it, I won't touch the keyboard..."
I think they do statistic hits, maybe defense/armor/attack, that kind of thing.
XIV: Misfit Stormrider - Kashuan
XI: Kujata - Razoredge (Long Retired) A possible retainer idea