Top spot flea control

Hi everyone - my cats (both desexed male DSHs) put up a fight when I
apply their monthly top spot anti-flea and worm treatment (but not as
much of a fight as trying to give them a pill!). That means I can't
neatly get the liquid onto the skin at the back of their neck; some
sticks to the fur around. I'm not absolutely sure how a top spot
works, or if this is a problem - I'm particularly concerned about
their swallowing it, or not being adequately protected from parasites.
They've never seemed to have a problem as a result, but I'm still
concerned. Can anyone give me some advice?

Thanks!
- Kate