If I may venture an opinion, I believe the translation is actually correct.
I think ki-ryoku-ni-yoru-seme (attack with the spirit) means exactly to attack with the spirit, to shatter your opponent's composure and thus create an opening. It is a mental ability (expressed through physical composure or with strong kiai, etc). I guess you can call it "to forcefully psych out your opponent".
The meaning of ki in kendo is pretty unambigously "spirit" or "mind". As evidenced in the phrase ki-ken-tai-ichi. If ki was a body skill then ki-tai ichi would be rather redundant, like saying "your internal skills and your body must arrive simultaneously"... huh?
I do not believe that particular concept (ki-no-seme) has anything to do with body skills. Not that internal skills is not relevant to kendo, I just think it's really not part of the formal curriculum.
Edit: Some obvious applications of internal skills in kendo I can think of would include resistance to tai-atari (body-checks), stronger kensen, ability to attack without visible windup... I'm sure experts can think of more then a newbie like me.