1. The person drinking it is a bro, and
2. He is in the presence of at least one other bro, and
3. There are no females present.
If all three conditions are met, then beer undergoes a transformation. Its existence as bro-soda goes from being merely potential, to full actuality.
Ethics: The term bro-soda should never be used in the company of females. For example, if in the presence of a lady-infested party, one bro says to another bro, "Hey, can you grab me a bro-soda?" then he has alienated all the females in his presence. Then, when the bro is alone with the female, she will bitch and moan, causing the bro to return to bro-night with the need to complain about his lady-bro. This restricts the capacity for bro-night to get to its full potential (which usually involves forgetting about the lady-bros).
Bro 2: "Have a bro-soda!"
Bro 1: "I Love Natty Lite!"