[Autumn]

You can check your SWA before and after equipping the skill in your character menu, the results seem to match up in a battle too when I tested against a Prinny.

Anyways the method I tested on was using a 41 WIL chara of mine with also like, 11 STR or so, by using a tome with Dualpower I noticed that my SWA had gone up by around 7 points, since my STR is so low I imagine its adding 10% Of my WIL instead.

Its just now that I remembered that the multiplier for SWA with a stat is 1.5 so its probably adding 10% WIL to Tomes (instead of 20% like I mistakenly mentioned.) and 10% STR to Swords, I also tested with a Yukijin afterwards to see if it was adding the same SWA or not, it was only adding 1 SWA which is probably because I had 11 STR.