The stated DMG increase of Fuufuu Whirlwind Kick from Sayu's C2 increases both Anemo Kick Hold DMG and Absorbed Element Kick DMG, by 3.3% every .5s tick
Using a Sayu with 80/80 r1 Sac GS, no artifacts (713 atk, 72 em), talent level 1 skill
In the first video, Sayu rolls at 24s, kicks at 26s, and the resulting ~2s roll deals 752 anemo and 263 pyro.
In the second video, Sayu rolls at 8s, kicks at 18s, and the resulting ~max length roll deals 1137 anemo and 398 pyro.
Rough napkin maths:
base anemo hold kick dmg: 716
base absorbed element kick dmg: 251