It is a simulator of a coax line with a slot down one side where a probe can be moved longitudinally to measure varying electric field strength. The probe has a detector that converts RF energy to DC voltage, so you can measure peaks and valleys using a voltmeter. For circuits that were extremely mismatched (or open or short circuited), the peaks and valleys are the most noticeable. The ratio of the peak voltage to the valley voltage was the most directly calculated piece of data you can get with a slotted line with "voltage standing wave ratio". with short- Circuit.