Finn Magic is the term given to the peculiar effect of Micky Finn's technology on the patrons of Callahan's who Finn has personally met. As a result they are immune to deadly physical forces, radiation, and rain unless they will it otherwise. The effects are highly selective, as Jake was unable to smoke a cigarette in the rain, but could smoke a joint.
The physical invulnerability only extends to forces which would be fatal. A common punch would not trigger it, but a gunshot would be stopped. The user can still feel the force, but at a much diminished amount (Erin reported that gunshots felt like tickling).