Why does Angier's In A Flash illusion make Borden so angry?

Angier's In A Flash illusion makes Borden angry because it is a copycat illusion to Borden's New Transporting Man illusion. However, Borden feels Angier has made it flashier and more exciting. Borden is most frustrated that he can't figure out how Angier is doing it, because Borden is positive Angier does not know Borden's secret and is not performing it the same way.