Tamil has a letter "ழ" with “zha” sound and that's the uniqueness of it. The concept of bending your tongue backward (ie., retro-flexing) is not prevalent in European languages. There is no equivalence for this alphabet in English similar to "psi" in Greek. “Zha” is just an approximation that sounds something like the original Tamil letter "ழ", they're not quite the same but the approximation works modestly well. This uniqueness is called "zhagaram" (ழகரம்) in Tamil Grammar which is celebrated by Tamil scholars and poets. The artist designs this GIF celebrating the uniqueness of this sound.