A diode array works by shining a laser through an absorbent sample and seeing the wavelength produced. As time progresses, the sample might change and information about absorbance over time is gained from the machine. In this case, information of the absorbance of the conversion between NADH to NAD+ was gained, as this conversion can be correlated to the rate of the reaction. If we see larger drops on absorbance over time, then the reaction is being inhibited.
Figure Six: How a diode array works (Instructables).