Mmmm.... turn fins? Perhaps he means a 'dagger plate' a vertical fin that should help prevent the boat from rolling and filpping on turns. Just theorizing I'm not sure though if two would be the answer, I wonder if they might actually hinder. With a single plate of the right size placed centrally the boat would be still be restrained from too much sideways roll yet not lose any directional control being able to pivot on the single plate, where as two plates would tend to act like two fixed rudders and be hinder to turning. I also fly planes and I agree with Dave M about fine control, if using a computerized radio dialing in both rates and expotential helps compensate for over enthusiastic waggling of the sticks and is nearly always required for fast craft particularly first time out. Anyway that sure looks like fun, that's a serious handful and on just 7 volts, well done!

Dagger boards are onl;y used on sail boats, turn fins are a fine edges plate, fitted to the transom, and adjustable in height/depth to the water, so it gives less "bite" when running straight, but digs in on turns to get the hull to bite in to the water.

A rudder is fitted, so there is no way of fitting one of these centrally my freind, they have to fit either side/one side of the transom.
"trimming the radio" does not cater for the movement on a fast boat, thats purely down to the person behind the sticks at the time!
The "end game" on the video was when the rudder dug in too deeply and the boat just spun around due to an "over steer" moment........... thast just what can happen when playing around setting up brushless systems.