I'm no hacker, but I'd say the server knows where the monsters are and who's hitting/killing them... The client might tell the server "I killed X many monsters" and the server would just send back "No you didn't." I dunno, I'm guessing that the server knows what you've killed, so if the client claims that you've killed something when you haven't, the server wouldn't believe it...
Just what I think... I might be wrong, as I said, I dunno how to hack...
Just what I think... I might be wrong, as I said, I dunno how to hack...