How does flea treatments work?

My dog has fleas and I treated her with Hertz once a month treatment thing....Anyway the directions told me to apply this gel/watery stuff from her shoulders down her back to the base of her tail...So I did but it kinda just stayed there on her back and never really coated her or anything. I am wondering just how in the world is this medicine supposed to work? Am I doing it wrong, or is there something better to use, cause I did this three nights ago and haven't seen any real difference in her itching.

DO NOT USE HARTZ!!! If you need to treat your dog for fleas, use the brands Advantage or Frontline, which can be found at any vet office and most pet shops. Hartz has killed thousands of dogs & cats. Throw it away or take it back to the store.Before you even consider using a Hartz flea product, just take a look at this: http://www.hartzvictims.org/There's actually a woman here on Yahoo Answers whose puppy was poisoned by the stuff just last night. The poor thing couldn't even walk. I don't know what the outcome was. I sure hope the puppy made it.