/debug @debug
should make a new custom window where all messages from and to the server for that connection are logged in exactly the way they are sent. This means that you see how normal messages are sent, what codes a /whois uses to send it's data, how channel joins are passed to your client and when the ping? pong! event happens. It also shows stuff that you could have halted in a script, for custom text layouts or for spam detection or whatever. It's also a nice tool to learn about the IRC protocol, it's not that difficult to understand what line means what mIRC action to happen
Type /debug off to stop this output
