Hi,
The guideline used by the spell is 15 = cripple a limb.
The spell uses then voice, and then add a target part.
I really don't get why the spell use a target part since the guideline, itself, already encompasses that.
The result is that the spell is the same level as a spell such as incantation of the milky eyes, which is based on a higher base, and that both wounds are not treated as the same level: milky eyes is healed as a heavy wound by magic, and twist as a medium wound.
Besides, both prevent someone to use the thing affected : milky eyes make someone unable to use a sense, which explains his level 20 and heal as a heavy, which is the same as the level 15 "normal" guideline. (one magnitude higher because of unability to use) for twist, it goes 2 levels higher and doesn't as much, follows the previous logic.
What do you think?