A Wounding Factor prevents rapid healing. Regeneration from a Healing Factor or medical treatment is reduced by the WF of an attack. +1WF costs 2R and counters +1HF. This is nasty when combined with persistent damage.
Example: Wounding Factor
A character has a quick draw stunt with her revolver. The stunt (1R) and the +2 weapon (1R) give her a standard attack of
/roll 4f +3Quick +2Stunt +2Revolver > +1 +1 -1 +0 +3 +2 +2 = +8 attack
This is not altered by a +2 wounding factor. It doesn't do damage; it prevents healing.
The target rolls a total of +4 to defend himself, taking a total of 4 stress. The target's HF+2 would normally take care of the stress in a few minutes, but it is countered by the +2WF. A separate stab wound worth 2PS and with no WF heals rapidly while the bullet wound stays open.