[YorkieMother]

Here is my limited understanding and someone can corret me if I am wrong.

If a fle jumps on your dog once the stuff is on it dead shortly but the flea can jump on at any time from outside or even if a place inside is infected.
So if you are having a flea problem the stuff you put on will be working but you need to clean up the bedding.
Also when playing outside the dog will pick up more fleas but will die.

JL