Question: When are the fins active?
About the article "Wakeboarding", section "Fins and placement".

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Active fin stabilizers are normally used to reduce the roll that a vessel experiences while underway or, more recently, while at rest.
The fins extend beyond the hull of the vessel below the waterline and alter their angle of attack depending upon heel angle and rate-of-roll of the vessel.
They operate similar to airplane ailerons.
Cruise ships and yachts frequently use this type of stabilizer system.
When fins are not retractable, they constitute fixed appendages to the hull, possibly extending the beam or draft envelope, requiring attention for additional hull clearances.
While the typical "active fin" stabilizer will effectively counteract roll for ships underway, some modern active fin systems have been shown capable of reducing roll motion when vessels are not underway.
Referred to as zero-speed or Stabilization at Rest, these systems work by moving fins of special design, with the requisite acceleration and impulse timing to create effective roll cancellation energy. 
The pectoral fins are normally held against the body, but when threatened the fins are expanded to startle potential predators which may include sea breams and mackerel.
The flying gurnard uses its pelvic fins to walk along the bottom of the ocean.
The oriental flying gurnard feeds on small bony fish, bivalves, and crustaceans. 
The closer the fins are placed towards the centre of the wakeboard, the quicker and better the wakeboard releases from the wake.
The farther out towards the tip and tail they are placed, the longer the wakeboard will stay hooked into the wake and it won’t release as well.
Long based finsTheir effect is almost the same as a short fin with a long base because they have a similar amount of surface area.
Long-based fins release better, give the wakeboard a loose, snowboard-like feel when riding flat through the water, and they hold up better on rails and ramps.
Moulded finsThese are just big channels in the board that act like fins and hold up on rails and ramps.
Moulded fins are slippery, but most boards have a removable centre fin.
Multi-finned set-upsThese capture the maximum edge hold and aggressiveness into the wake and through the wake.
Canted side finsThese are fins that lean out on an angle.
These fins are not as active when the wakeboard is riding flat through the water, but the more the rider leans on edge, the more the wakeboard hooks up.
The inside fin digs while the outside lifts, creating leverage to help the wakeboard edge hard.
Great for 50-50 grinds, nose presses and tail presses.
Cupped side finsThey have the same effect as canted fins but add more of a push-pull effect.
The cupped fin allows for the use of a smaller fin but still gets the hold of a bigger fin due to the increased surface area of the cupped side of the fin.
These fins are very deceiving – they look small and loose but really aren’t.
No FinsSome riders prefer to ride finless, as some boards are specifically designed for cable parks or other uses, some uses of which can benefit from a finless design.
Ship stabilizers are fins or rotors mounted beneath the waterline and emerging laterally.
In contemporary vessels, they may be gyroscopically controlled active fins, which have the capacity to change their angle of attack to counteract roll caused by wind or waves acting on the ship. 
Fins allow the player to swim faster through the water.
A wide range of fins are used in the sport but large plastic/rubber composite fins or smaller, stiffer fibre glass or carbon fibre fins are commonplace at competitions.
Once again they must have no unnecessarily acute or sharp edges, nor buckles.