When you think you can sit behind your computer screen and write whatever you want, to whomever you want. These are not things you would say the person if you were face to face. That would require having balls when you're away from your computer...which you don't. Internet balls are only obtained while behind the screen and keyboard.
"Liz has internet balls. She wrote a long winded post about my wife being a drugged out zombie. I would like to see her say that stuff to her face!"
n. - the metaphorical "testicles", courage, that suddenly sprouts when you bitch someone out on the internet in a way which you would not if you were face to face.
"Dude, were you on facebook last night?! Kevin grew internet balls and made himself look like a huge tool!"
People who are braver behind emails than in person
Justin had internet balls when writing that scathing email, but when I confronted him in person and he acted like a baby back bitch.