Dynamic apnea with fins is made up of two main types of events: to swim as far as possible underwater horizontally breath-holding and to swim as quickly as possible a certain distance breath-holding or alternating apnea and recovery. Whether in bi or monofin the equipment used will not be the same.
To travel the longest distance, some rely on their ability to hold their breath for a long time, so it takes rare very slow kicks of great amplitude with a very flexible blade. This is the 'slow' option. Conversely, others choose the so-called 'fast' option by relying on their ability to move quickly in apnea with a higher frequency, they will prefer a stiffer blade. For the sprint races we will also prefer harder blades.
Then to choose the right length, everything will depend on its level, its objectives and the size of the swimming pool because you need a good technique to keep your momentum with long fins after turns and push-offs from the wall.
Finally the footpocket is very important. If it’s uncomfortable, it will put a strain on your concentration and relaxation, and a footpocket that doesn’t transfer efficiently the movement to the blade will cause a lot of energy loss.