Motorists exiting south on Speer passed a beam of infrared light at hubcap level that monitored tailpipe emissions for a half-second. Moments later, a large sign informed each driver about her or his vehicle's performance in qualitative terms ("good," "fair," or "poor"). The technique gives readings for carbon monoxide, hydrocarbons, and nitric oxide as a ratio of their emission levels to that of carbon dioxide; this allows for accurate sampling despite varying environmental conditions.

Stedman's device is based on many of the same principles as the dynamometer used by Envirotest, the state's official emissions tester. However, Stedman samples emissions on the fly rather than capturing them in a box for analysis, as do the official tests. Stedman's invention can sample hundreds of cars in an hour, a much faster pace than the official tests allow.