There are a series of weaponskills that have "additional effects" and receive this description. I believe this is a comprehensive list:
Brain Shaker
Flash Nova
Tachi:Hobaku
Tachi: Ageha*
Shoulder Tackle
Shijin Spiral
Smash Axe
Gale Axe
Bora Axe
Leg Sweep
Flat Blade
Death Blossom^
Mordant Rime
Anyway, this description is misleading because it implies that the chance an additional effect will be inflicted always depends on your TP when you use the weapon skill. This is not the case, and if you fight a low enough level monster then you can expect these effects will land every single time, entirely reliably (except for the *d one, which will never land because it is broken and maybe the ^ one which I haven't tested).
The description should probably be something like, "Additional effect accuracy varies with TP." This implies that the accuracy may increase with additional TP, but everyone understands that against weak monsters you will already have enough accuracy to cap hit rate. It would be analogous to the Penta Thrust description.
Thanks!