It makes illegal acts worse. 9th circuit (where Russ now is) defined the rule as "this principle renders defendants liable for any physical injury they cause, no matter how unforeseeable, once they inflict harm on a plaintiff's body". As an example, they gave "It is as if a magic circle were drawn about the person, and one who breaks it, even by so much as a cut on the finger, becomes liable for all resulting harm to the person, although it may be death.... The defendant is held liable for unusual results of personal injuries which are regarded as unforeseeable, such as ... heart disease.... The defendant of course is liable only for the extent to which the defendant's conduct has resulted in an aggravation of the pre-existing condition, and not for the condition as it was; but as to the aggravation, foreseeability is not a factor".
Another example, if I hit you, but you have osteoporosis, and then you die because all your bones collapse, I am more liable than had you not had that condition.
The citation is from Gibson v. County of Washoe, Nevada, 290 F.3d 1175 (9th Cir. 2002)