It's just saving a list of already created fontstrings into the f2.Fonstrings table as {left, right} pairs and re-using them if they exist or creating additional if required. It's just a demonstration so it's not handling things like hiding fonststrings if subsequent clicks contain a shorter list etc.
This would really be better as a scroll list if you're likely to have more entries than you can display vertically in the frame.
This contains a link in the OP to one way this might work.