Okay. Imagine you have a Sceptile against any Pokémon of your choice. Now, let's say it attacks that Pokémon, and leaves them with almost half the health they originally had. Note the word almost. Anyway, When Sceptile attacks again, the opponent survives with one Hit Point, which wouldn't be possible if it weren't for False Swipe. So no matter how many times you attack, False Swipe will always leave a Hit Point for the opponent to hang onto.