MIDI Monitor is used to watch what MIDI events are streaming through the system. After placing this FX in the desired location, left click it to watch MIDI events in the Selection 1 view.

The initial state After hitting a few keys

The Event Types button is used to select which types of MIDI events to display; by default most everything is on. The Show hidden box can be turned on to display 'hidden' events, which are typically system-generated and not visible in the track editor. For example, placing a controller of 0 at measure 1 and 127 at measure 4 will generate all the intermediary events necessary to smoothly transition between the two values. All these intermediary events are 'hidden' events, and can be viewed by engaging this parameter.