Killing hands on astral opponents Options

[Machiavelli]

Example: troll ghoul is attacking an wholly astral opponent with an unarmed blow. Because he is attacking somebody in astral space, he should have to use astral combat+willpower, causing Charisma/2 damage. So far so good. But what if this troll has killing hands as an adept power? This power is usable in astral combat, but for what reason? In astral combat you can choose if you want to do physical or mental damage anyway. There is also no ItnW in astral space and it doesn´t negate mystic armor. That you can take your close combat skill instead of your astral combat skill is also no option, because the skill of astral combat covers the unique ways of damaging opponents that can use the astral movement options. So might this power possibly help you to cause Str/2 damage? What exactly does it do?