This is an enhancement for Spirit Form present in GURPS Spirits and Hellboy Sourcebook and Roleplaying Game
GURPS Undead free excerpt at http://www.sjgames.com/gurps/books/Undead/excerpts.html
- Specter characters can purchase Extra Fatigue and Strong Will freely with earned character points. With the GM's permission, they can also acquire the Spectral Touch enhancement (p. 54) [40] on their Ghost Form. This is mostly of use to specters who know many spells, as it lets them attack with Deathtouch, Paralyze Limb, Wither Limb, etc., while safely insubstantial. This is a powerful ability, and the GM may choose to control it or forbid it entirely.
This is similar to Affects Substantial but it only works like a Melee Attack