It's not used often---only when the conversation seems to start hurting feelings or is looking like it's heading to something really controversal or against the rules. It's used to keep us playing nice. I also think that the person that started the thread can request that it be locked. You can pm Patrice and ask her when, why and how it's used. She can explain it better since she's a moderator. I think sometimes is used when a subject has been hashed to death too but I could be wrong.