In a nutshell, she has no powers but doesn't hesitate to tackle trouble when it literally arrives on her doorstep. And she makes her stand while wearing a handy Spider Woman costume, complete with hideous spider head (all the better for preying on the cowardly criminal element).
Her reasons for having the costume are never explained, but her own words imply this isn't the first time.
"I'd better be prepared for trouble!" she thinks as she rushes toward home, where she quickly dons the costume. "I'm afraid to use a gun, but they'll be afraid, too, when they see Spider Woman."
If there really is an untold origin, it may be related to the death of Helen's father, referred to throughout this story (death of a loved one being a common motivator for costumed vigilantes).
Regardless, Spider Woman comes and goes with nary a ripple on the public consciousness.