The original idea of that master blacksmith was to create a pair of shoes with something like the 'sword riding feature', which would allow it to work as 'flying shoes'. However, they ultimately failed, and obtained a super weakened version of those 'flying shoes', which were then transformed into 'face slapping shoes'.
Actually, there were many cultivators that had carried out similar experiments. A lot of cultivators dreamt of flying before reaching the Fourth Stage Realm. For example, Great Master Profound Principle had created a half-finished 'sword controlling technique', and Medicine Master that 'artificial insect' that could allow one to turn into a hot air balloon.
After hearing the explanation, Song Shuhang felt that this Face Slapping Flip-flop was rather interesting. "Shopkeeper, how much is it for this face slapping flip-flop?"