In practice what does it do? "the target cannot defend itself" as in...? Lower AC, all attacks automatically hit? I'm trying to determine if this is a good spell to learn.

I never used it in 1 due to my MP'er thing, and the two servers I run on, it's really a wasted slot, but, in NWN2, it caused the opponent I hit with it to fall prone on the floor. So, if I were to guess, I'd say it would apply a daze effect, or possibly a knockdown.