Pete: I believe the backside acts as a resonator for a banjo: "bouncing " the sound waves forward, making it louder. (The backside is looser so it can vibrate more) Thus, it’s probably not too bad even with a tear. If it’s a tiny tear, superglue would be a good way to prevent it from tearing more.
Cana: Actually, the place where it tore is the thinner part of the skin (called Daki). The thin daki side of the skin is positioned under the ichi no ito to give a more “complex” (as they say) tone. Anywho, because it’s thinner, that area is usually the first place to tear, I think.