This option is very powerful and gives the ability to run a lua script upon each log message received. Lua is compact and highly efficient scripting language, which can be used as a sophisticated log analyzer. TailExpert provides a new logview pane to the Lua script for output, but it is also supported from lua to use .NET components to create new complex windows or dialogs like listboxes, checkboxes etc. to display analysis results, an example is provided in the lua directory under the installation directory. Lua can be set to passthrough all messages (default) however it can also be set that messages are capured by lua and then your script is responsible to forward messages to the originating logview.
Lua scripting is available to both file watch and syslog type of watch sessions.