The result of too much knee rotation: drag
This is just a snippet of finning advice, taken from Glass and Water: The Essential Guide to Freediving for Underwater Photography…
“A classic mistake is to make too much movement at the knee joint. When this happens you will look as if you are riding an imaginary underwater bicycle. Conversely, too much movement in the hip and too little in the knee will produce a stiff looking stroke which will rock the pelvis. Pivoting all three points in unison enables a fluid movement that uses all the lower muscle groups and tendons. This will provide more power and produce less fatigue than overuse of one or two.
The amplitude of your stroke is the measurement between the highest and lowest points that your legs reach during a fin cycle. This needs to be approximately fifty to sixty percent of the maximum extent possible. Although a full, wide kick may feel quite powerful, it also acts as a partial brake against the oncoming flow of water. In regard to the frequency of the kick, again less is more. Too much and your oxygen consumption will increase. You can of course fin too slowly and end up losing speed and momentum. Fin kick amplitude and frequency are examined in more detail in Chapter 9: Hydrodynamics of Glass and Water.”
Mark Harris is the author of Glass and Water: The Essential Guide to Freediving for Underwater Photography which covers much more on finning advice and plenty more besides. He is a former UK champion freediver, record-holder and instructor who has also coached and judged at freediving competitions. He has consulted on and taught students how to freedive for roles in both television and film. For almost a decade, he ran London’s main club, London Freediving. Mark is a member of the British Society of Underwater Photographers. You can see more of his work on his website subscenic.co.uk