I'm new to using this mod and I got the Featherlight sword, and I need the Calculated Fall effect to perform the special attack with Featherlight. The Effects description of the wiki only tells me what the effect does, and not how to get it. Can anyone tell me how to get this effect for the attack?

Certain weapons such as the Comet Spear and Darkin Blade grant it when using their ability.

Only the Comet Spear and Darkin Blade gives you the effect. I'll add info to the wiki so this info is more accessible.