I liken virtual balls to liquid courage. The anonymity and physical distance of the Internet affords many people the opportunity to present themselves as much bolder and brazen than they could otherwise muster during in-person communication.
For example, most of us no doubt have seen threads on listservs or blogs where someone who's a wuss in real life tries to shake other discussants through written aggression or by attempting to dominate the discussion with fancy jargon. Here, the person is mouthy or domineering online because it’s a safe venue. He or she has BIG virtual balls (but itty bitty balls in real life).