If Lock Log Files is turned off, log files are opened, written to, and closed each time a new line is written to the log. If its on, then log files are opened/written to and only closed when you close the window or turn logging off.
Description of this particular option is detailed in the versions.txt
for 5.1. Just as an FYI for those who dont know, you can d/l a complete versions.txt
for all releases to date at http://www.mirc.co.uk/versions.txt