This is called resonant frequency (in the torsional mode). Resonant frequency is sqrt(k/m) with k=stiffness and m=mass. Therefore, two masts with the same stiffness can actually have different resonant frequencies (wobble) if the mounted wings are different weights. While this test can be insightful into stiffness, it is more accurate to twist the mast statically to remove any effects of wing weight or fuselage length. To do this, twist the mast with a known force and record deflection. This will tell you the stiffness of the mast by eliminating other variables that affect dynamic test results.
When the mast is in the water, it is effectively damped. Damping is not the same as stiffness. A carbon mast with the same stiffness as an aluminum mast will have better damping characteristics due to the intrinsic properties of the composite material. You might feel this in the water with less vibration, but it's still the same stiffness as the aluminum mast. It also may wobble more or less in this test, but again both masts have the same stiffness. In the case of Project Cedrus, the rubber edges actually add additional damping to further reduce vibration.
Torsional stiffness is critical when yawing the wings. For example, tight turns in the surf, a quick upwind tack. The input comes from the rider/board, not the wing[tips] or vertical stabilizer if applicable. Inertia and drag prevent wings from yawing, so a stiffer mast helps. Furthermore, it can reduce the likelihood of ventilation by reducing the amount of twist in the mast at aggressive angles of attack. A twisting mast can cause flow separation/stalling. Ventilation is very complex, and torsional stiffness is only one of the factors.
Oddly the new Axis HM carbon mast has lower torsional stiffness than their aluminum mast. Devon with Wake Thief did a really good test of it, with proper loadcases (not just hanging weight of the end of the mast).
https://www.youtube.com/watch?v=EV6O5vQep8Y From a technical standpoint, there's really no excuse for a carbon mast have lower torsional or bending stiffness than an aluminum mast. Carbon is much stiffer than aluminum, so when properly engineered it should be superior from every standpoint. I wrote about this here:
https://projectcedrus.com/general/solid ... sites-101/
It's nice to see the industry recognizing the importance of stiffness. However proper testing and engineering is critical to maximizing it, and based on how some brands continue to test their masts, they will be lead down the wrong path of design...