Gill, this is what it says on their website.
A. Many spot-on products remain on the surface of the pet. Fleas take up these products when they jump onto a treated animal either by mouth or through their surface (called the cuticle). Fleas do not need to suck blood for the product to start working. Consult the product label for more information.
I'm going to ask my vets about it, worried to give Wordy advocate after him being so poorly, it may have been nothing to do with that, but it is a powerful product.