In discussing how fins work in a multi fin pod, there are two types of flow, laminar and turbulent, I do believe.
No doubt most everyone has seen the eddies and rolling off the side of your boat as you are wakesurfing. It can also bee seen off the back when your boat is unladen and you're blasting somewhere. That is a layer of turbulent, rolling, swirling water thats releasing off the hull of the boat.
That layer of turbulence is being caused by viscous drag or surface drag.
On your board you go from laminar flow at low speeds (paddling speed in the ocean) to turbulent flow at high speeds. Turbulent flow is not a bad thing because that layer of turbulence insulates the bottom of your board from the main body of still water and acts almost like ball bearings under your board. It helps speed.
While turbulent flow is great for the bottom of your board, its a nightmare for your fins.
If a plane flys through turbulence it can lose altitude rapidly as the wings can't function as effectively in turbulence. Similarly if your fins have to deal with turbulence they don't function as well, lose hold, encounter more drag, especially at angles of attack (like turns up and into the wake).
A cut-away in the trailing edge lets the turbulent water pass under the fin, so less of the fin has turbulent water pushed up against it in turns. The part of a normal fin (without a cut-away) in contact with the board won't act efficiently, have more drag, less hold. The cut away allows the fin to operate in a zone away from the board in cleaner (less turbulent) water, allowing it to perform more efficiently.
This theory is seen frequently in nature on the fins of fish and on the wings of some birds.